Geological scale. time, showing the sequence and subordination of the stages of development of the earth's crust and organic. the world of the Earth (eons, eras, periods, epochs, centuries). The sequence of deposits is reflected in the so-called. stratigraphic scale, units to swarm ... ... Biological encyclopedic dictionary

Geological scale- (a. geological dating, geochronological scale; n. geologische Zeitrechnung; f. echelle geochronologique; i. escala geocronologica) follow. a number of geochronological equivalents of common stratigraphic. subdivisions and their taxonomic ... ... Geological Encyclopedia

geochronological scale- — Topics oil and gas industry EN geologic time scale …

Geological scale- see Art. Geochronology… Great Soviet Encyclopedia

Geochronological scale of the Phanerozoic- (duration 570 million years) Eras and their duration Periods Beginning of periods, million years ago Duration of periods, million years Development of life Cenozoic (67 million years) Anthropogenic Development of mankind. Neogene Appearance of man ... ... Beginnings of modern natural science

geochronological scale- The scale of geological time, showing the sequence and subordination of the main stages of the geological history of the Earth and the development of life on it. [Glossary of geological terms and concepts. Tomsk State University] Topics geology ... Technical Translator's Handbook

SCALE GEOCHRONOLOGICAL (GEOHISTORICAL)- scale of relative geol. time, showing the sequence and subordination of the main stages of geol. the history of the Earth and the development of life on it. It is the result of the analysis and synthesis of all data of the stratigraphic scale and, accordingly ... ... Geological Encyclopedia

GEOCHRONOLOGICAL PALEOMAGNETIC SCALE,- Cox, Doell, Dalrymple, 1968, based on reversals of the Earth's magnetic field that have occurred many times in geol. past. Developed for the last 4.5 million years of the Cenozoic. The main units of the Sh. G. p. are epochs (lasting about 1 1.5 million years ... Geological Encyclopedia

geochronological scale- geochronological scale geological dating, geochronological scale geologische Zeitrechnung last series of geochronological equivalents of global stratigraphic subdivisions and their taxonomic agility. First geochronological scale for ... ... Girnichiy encyclopedic dictionary

Selenochronological scale- Age of some areas on the Moon: 1 Age of craters (a Nectar, b Imbrian, c Eratosthenes, d Copernican) 2 Age of the seas (a Prenectar, b Nectar, c Early ... Wikipedia

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Four chronograms are presented, reflecting different stages of the Earth's history on a different scale.

The top diagram covers the entire history of the earth;

The second - Phanerozoic, the time of the mass appearance of various forms of life;

The third is the Cenozoic, the period of time after the extinction of the dinosaurs;

The lower one is the Anthropogen (Quaternary period), the time of the appearance of man.

Millions of years

The largest subdivision is the eon, of which 3: 1 stands out) Archaean(Greek "archeos" - the oldest) - more than 3.5-2.6 billion years; 2) Proterozoic(Greek "proteros" - primary) - 2.6 billion years - 570 million years; 3) Phanerozoic(Greek "Phaneros" - explicit) - 570 - 0 million years. Eons are subdivided into eras, and they, in turn, into periods and epochs (see the geochronological scale).

The Phanerozoic eon is subdivided into eras: Paleozoic(Greek "paleos" - ancient, "zoo" - life) (6 periods); Mesozoic(Greek "mesos" - middle) (3 periods) and Cenozoic(Greek "kainos" - new) (3 periods). 12 periods are named after the area where they were first identified and described - Cambrian - the ancient name of the Wales Peninsula in England; Ordovician and Silurian - by the name of the ancient tribes who also lived in England; Devon - in the county of Devonshire, again in England; carbon - for coal; perm - in the Perm province in Russia, etc.

Geological periods have different durations from 20 to 100 million years. As for the Quaternary or anthropogen(Greek "anthropos" - a man), then it does not exceed 1.8-2.0 million years in duration and has not yet ended.

Attention should be paid to the stratigraphic scale, which deals with deposits. It uses other terms: eonoteme (eon), eratheme (era), system (period), department (epoch), tier (age). Therefore, we say that in during the carboniferous period deposits of coal were formed", but "the coal system is characterized by the spread of coal-bearing deposits". In the first case, we are talking about time, in the second - about deposits.

All divisions of the geochronological and stratigraphic scales of the period-system rank are indicated by the first letter of the Latin name, for example, Cambrian є, Ordovician - O, Silurian - S, Devonian - D, etc., and epochs (divisions) - numbers - 1.2, 3, which are placed to the right of the index at the bottom: Lower Jurassic J1, Upper Cretaceous - K2, etc. Each period (system) has its own color, which is shown on the geological map. These colors are generally accepted and cannot be replaced.

The geochronological scale is the most important document that satisfies the sequence and time of geological events in the history of the Earth. It must be known without fail and therefore the scale must be learned from the very first steps of studying geology.

Isotopic methods for determining the age of minerals and rocks

After the discovery in 1896 by the French physicist A. Becquerel of the phenomenon of radioactive decay, it became possible to establish the age of minerals and rocks. It was also found that the process of radioactive decay occurs at a constant rate, both on our Earth and in the solar system. On this basis, P. Curie (1902) and, independently of him, E. Rutherford (1902) suggested the possibility of using the radioactive decay of elements as a measure of geological time. So science at the beginning of the 20th century approached the creation of clocks based on radioactive natural transformations, the course of which is independent of geological and astronomical phenomena.

Question number 3. geodynamic processes. Geological disturbances

Plate tectonics - modern geological theory

A decisive contribution to the modern geological theory of lithospheric plate tectonics was made by the following discoveries: 1) the establishment of a grandiose, about 60 thousand km system of mid-ocean ridges and giant faults crossing these ridges; 2) detection and interpretation of linear magnetic anomalies of the ocean floor, making it possible to explain the mechanism and time of its formation; 3) establishing the location and depths of hypocenters (foci) of earthquakes and solving their focal mechanisms, i.e. determination of the orientation of stresses in the centers; 4) the development of a paleomagnetic method based on the study of the ancient magnetization of rocks, which made it possible to establish the movement of continents relative to the Earth's magnetic poles.

The lithospheric plate is a large stable area of ​​the earth's crust, part of the lithosphere. According to the theory of plate tectonics, lithospheric plates are limited by zones of seismic, volcanic and tectonic activity - plate boundaries. There are three types of plate boundaries: divergent, convergent and transformative.

Only three plates can converge at one point. A configuration in which four or more plates converge at one point is unstable, and quickly collapses over time.

There are two fundamentally different types of earth's crust - continental crust and oceanic crust. Some lithospheric plates are composed exclusively of oceanic crust (an example is the largest Pacific plate), others consist of a block of continental crust soldered into the oceanic crust.

Lithospheric plates are constantly changing their outlines, they can split as a result of rifting and solder, forming a single plate as a result of collision. Lithospheric plates can also sink into the planet's mantle, reaching deep into the outer core. On the other hand, the division of the earth's crust into plates is ambiguous, and as geological knowledge accumulates, new plates are distinguished, and some plate boundaries are recognized as non-existent. The outlines of the plates change over time. This is especially true for small plates, for which geologists have proposed many kinematic reconstructions.

More than 90% of the Earth's surface is covered by the 14 largest lithospheric plates.

The main idea of ​​the new theory was based on the recognition of the separation of the lithosphere, i.e. the upper shell of the Earth, including the earth's crust and upper mantle to the asthenosphere, into 7 independent large plates, not counting a number of small ones.

These plates in their central parts are devoid of seismicity, they are tectonically stable, but seismicity is very high along the edges of the plates, earthquakes constantly occur there. Consequently, the edge zones of the plates experience large stresses, because. move relative to each other.

The main lithospheric plates (according to V.E. Khain and M.G. Lomize): 1 - spreading axes (divergent boundaries),2 – subduction zones (convergent boundaries),3 - transform faults,4 - vectors of "absolute" movements of lithospheric plates. Small plates: X - Juan de Fuca; Ko - Coconut; K - Caribbean; A - Arabian; CT - Chinese; I - Indochinese; O - Okhotsk; F - Philippine

Having determined the nature of the stresses in the earthquake sources at the edges of the plates, it was possible to find out that in some cases this tension, i.e. the plates diverge and this happens along the axis of the mid-ocean ridges, where deep gorges - rifts (English "rift" - crevice) are developed. Similar boundaries marking the zones of divergence of lithospheric plates are called divergent(English divergence - divergence).

Shell structure of the Earth

Modern seismicity, volcanism and plate boundaries

Types of boundaries of lithospheric plates:1 - divergent borders. Opening of oceanic rifts that cause the spreading process: M - Mohorovichic surface, L - lithosphere;2 - convergent boundaries. Subduction (immersion) of the oceanic crust under the continental one: thin arrows show the mechanism of extension-compression in earthquake hypocenters (asterisks); P - primary magmatic chambers; 3 – transform boundaries; 4 - collision boundaries.

Divergent boundaries

Convergent (subduction) boundaries: the interaction of the oceanic plate with the continental one and the interaction of oceanic plates

The thrusting of the oceanic plate on the continental - obduction

Convergent boundaries (collision and interaction of continental plates)

Transform borders

The location of the axial parts of the mid-ocean ridges. Are major divergent boundaries

Plate boundaries, directions and velocities of plate movement, centers of modern seismic and volcanic activity

Kinematics of lithospheric plates

At other plate boundaries in earthquake sources, on the contrary, the setting of tectonic compression was revealed, i.e. in these places, lithospheric plates move towards each other at a speed reaching 10-12 cm/year. Such borders are called convergent(English convergence - convergence), and their length is also close to 60 thousand km.

There is another type of lithospheric plate boundaries, where they move horizontally relative to each other, as if shifting, as evidenced by the situation of shearing in earthquake sources in these zones. They got the name transform faults(eng. transform - transform), because. transmit, transform movements from one zone to another.

Some lithospheric plates are composed of both oceanic and continental crust at the same time. For example, the South American single plate consists of the oceanic crust of the western part of the South Atlantic and the continental crust of the South American continent. Only one, the Pacific Plate, consists entirely of oceanic-type crust.

Modern geodetic methods, including space geodesy, high-precision laser measurements and other methods, have established the speed of movement of lithospheric plates and it has been proven that oceanic plates move faster than those that include a continent, and the thicker the continental lithosphere, the lower the speed of plate movement.

The generally accepted point of view of the movement of lithospheric plates is the recognition of the convective transfer of mantle matter. The superficial expression of this phenomenon is the rift zones of the mid-ocean ridges, where the relatively warmer mantle rises to the surface, undergoes melting, and magma erupts in the form of basalt lavas in the rift zone and solidifies.

Origin of band magnetic anomalies in the oceans. A and C – time of normal, B – time of reverse magnetization of rocks:1 - oceanic crust2 - upper mantle3 – rift valley along the axis of the mid-ocean ridge,4 - magma,5 – band is normal and6 – back magnetized rocks

Further, basaltic magma again intrudes into these frozen rocks and pushes older basalts in both directions. And this happens many times. At the same time, the ocean floor, as it were, grows, grows. Such a process is called spreading(English spreading - deployment, spreading). Thus spreading has a velocity measured on both sides of the mid-ocean ridge axial rift.

The growth rate of the ocean floor ranges from a few mm to 18 cm per year. Linear magnetic positive and negative anomalies are located strictly symmetrically on both sides of the mid-ocean ridges in all oceans. Everywhere we see the same sequence of anomalies, in each place they are recognized, all of them are assigned their own serial number.

In other words, on both sides of the mid-ocean ridge, we have two identical "records" of magnetic field changes over a long period of time. The lower limit of this "record" is 180 million years. Ancient oceanic crust does not exist. Such a process is spreading.

Thus, the oceanic lithosphere builds up on both sides of the ridge, as it moves away from which it becomes colder and heavier and gradually descends, pushing through the asthenosphere.

The edge of the plate, under which the ocean subducts, cuts the sediments accumulated on it, like a scraper or bulldozer knife, deforms these sediments and grows them to the continental plate in the form accretionary wedge(English accretion - increment). At the same time, some part of the sedimentary deposits sinks together with the plate into the depths of the mantle.

In different places this process goes in different ways. Thus, off the coast of Central America, where wells have been drilled, almost all sediments move under the continental margin, which is facilitated by the ultrahigh pressure of water contained in the pores of the sediments. Therefore, there is very little friction. In a number of other places, the subducting oceanic lithospheric plate destroys and erodes the edge of the continental lithosphere and drags its fragments into the depths.

You should also mention the collision or collisions two continental plates, which, due to the relative lightness of the material that composes them, cannot sink under each other, but collide, forming a mountain-fold belt with a very complex internal structure. So, for example, the Himalayan mountains arose when the Hindustan plate collided with the Asian plate 50 million years ago.

This is how the Alpine mountain-fold belt was formed during the collision of the African-Arabian and Eurasian continental plates.

Relative movements of lithospheric plates and distribution of spreading rates in rift zones MOR (cm/yr): 1 – divergent and transform plate boundaries;2 – planetary compression belts;3 – convergent plate boundaries

The calculated absolute and relative movements of lithospheric plates since the beginning of the breakup of Pangea, i.e. from 180 million years ago are well known and highly accurate.

A picture of the opening of the Atlantic and Indian Oceans has been recreated, which continues to this day at a rate of about 2.0 cm per year. The possibility of some rotation of the Earth's lithosphere with respect to the lower mantle in the western direction has been elucidated, which makes it possible to explain why subduction conditions are not the same on the western and eastern active margins of the Pacific Ocean and a well-known asymmetry of the Pacific Ocean arises with back-arc, marginal seas and chains of islands in the west and the absence of such in the east.

The theory of lithospheric plate tectonics for the first time in the history of geology is global in nature, because it concerns all regions of the globe and makes it possible to explain their history of development, geological and tectonic structure.

GEOCHRONOLOGICAL SCALE (relative geological time scale), a sequence of subordinate geochronological units of various ranks, arranged in chronological order and covering the entire geological history of the Earth. The basis of the geochronological scale was the general stratigraphic scale developed by many years of practice mainly by European geologists in the 19th century, which is still being refined today. The time intervals during which deposits accumulated, taken as standards (stratotypes) of common stratigraphic units, were taken as common geochronological units. Geochronological subdivisions - akron, zones, era, period, epoch, century, phase - correspond to stratigraphic subdivisions - acrotheme, eonoteme, eratheme (group), system, department, stage, zone. The names of geochronological subdivisions indicate the relative geological age of objects of geological research.

Initially, the geochronological scale, combined with the stratigraphic scale, was compiled and approved at the 2nd session of the International Geological Congress in Bologna (Italy) in 1881 as a sequence of periods divided into epochs; since then it has been constantly improved. In this scale, the history of the Earth was divided into four eras, justified by the global stages in the development of the organic world, in connection with which their names were proposed: Archean, or Archeozoic, - the era of ancient life; Paleozoic - the era of ancient life; Mesozoic - the era of middle life; Cenozoic - the era of new life. In 1887, the Proterozoic - the era of primary life - was singled out from the Archean era. Later, it became necessary to single out subdivisions larger than eras - eons, which included the Archean, Proterozoic and Phanerozoic (united the Paleozoic, Mesozoic and Cenozoic eras).

In Russia, in the Precambrian part of the geochronological scale (1992), taking into account the enormous duration of the Precambrian (86% of the entire geological history), subdivisions of an even larger level were distinguished - the Acrons, to which the Archaean and Proterozoic were elevated. At the beginning of the 21st century, the geochronological scale has the form presented in the tables. There are 12 periods in the Phanerozoic eon: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian (they make up the Paleozoic era); Triassic, Jurassic, Cretaceous (Mesozoic era); Paleogene, Neogene and Quaternary (Cenozoic era). The names of the periods correspond to the names of the systems, which are mainly given by the name of the area where the systems were first identified and most fully described. Smaller divisions than periods in the geochronological scale are epochs, of which there are two (early and late) or three (early, middle and late). In some cases, epochs have their own names (for example, epochs of the Paleogene and Neogene periods). The next, more subdivisions of the geochronological scale are centuries and their subordinate phases. All period boundaries and most of the epochs are dated by isotopic methods.

Geological scale adopted in Russia, million years*

In Russia, the geochronological scale, combined with the General Stratigraphic Scale, was approved by the Interdepartmental Stratigraphic Committee (MSC) and included in the Stratigraphic Code (1992), which was supplemented in 2000. The international stratigraphic (geochronological) scale, developed by the International Commission on Stratigraphy and approved by the International Union of Geological Sciences (2004), in the Phanerozoic part differs from the domestic scale in the absence of the Quaternary period, which is included in the Neogene period, and in a different division into epochs and centuries of periods of the Paleozoic era; Acrons are not distinguished in the Precambrian part of the scale, while the Archean and Proterozoic are considered as subdivisions of a lower rank - eons, which have a different division into eras and periods than in the Russian geochronological scale.

Lit.: Stratigraphic codex. SPb., 1992; Additions to the Stratigraphic Code of Russia. St. Petersburg, 2000; A geologic time scale // Ed. by F. M. Gradstein, J. G. Ogy, A. G. Smith. 3rd ed. Camb.; N.Y., 2004.

Geological scale. time, showing the sequence and subordination of the stages of development of the earth's crust and organic. the world of the Earth (eons, eras, periods, epochs, centuries). The sequence of deposits is reflected in the so-called. stratigraphic scale, units to swarm ... ... Biological encyclopedic dictionary

- (a. geological dating, geochronological scale; n. geologische Zeitrechnung; f. echelle geochronologique; i. escala geocronologica) follow. a number of geochronological equivalents of common stratigraphic. subdivisions and their taxonomic ... ... Geological Encyclopedia

geochronological scale- — Topics oil and gas industry EN geologic time scale …

See Art. Geochronology… Great Soviet Encyclopedia

Geochronological scale of the Phanerozoic- (duration 570 million years) Eras and their duration Periods Beginning of periods, million years ago Duration of periods, million years Development of life Cenozoic (67 million years) Anthropogenic Development of mankind. Neogene Appearance of man ... ... Beginnings of modern natural science

geochronological scale- The scale of geological time, showing the sequence and subordination of the main stages of the geological history of the Earth and the development of life on it. [Glossary of geological terms and concepts. Tomsk State University] Topics geology ... Technical Translator's Handbook

Scale of relative geol. time, showing the sequence and subordination of the main stages of geol. the history of the Earth and the development of life on it. It is the result of the analysis and synthesis of all data of the stratigraphic scale and, accordingly ... ... Geological Encyclopedia

Cox, Doell, Dalrymple, 1968, based on the reversals of the Earth's magnetic field that have repeatedly occurred in geol. past. Developed for the last 4.5 million years of the Cenozoic. The main units of the Sh. G. p. are epochs (lasting about 1 1.5 million years ... Geological Encyclopedia

geochronological scale- geochronological scale geological dating, geochronological scale geologische Zeitrechnung last series of geochronological equivalents of global stratigraphic subdivisions and their taxonomic agility. First geochronological scale for ... ... Girnichiy encyclopedic dictionary

Age of some areas on the Moon: 1 Age of craters (a Nectar, b Imbrian, c Eratosthenes, d Copernican) 2 Age of the seas (a Prenectar, b Nectar, c Early ... Wikipedia

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• Earth is a restless planet: Atmosphere, hydrosphere, lithosphere: A book for schoolchildren ... and not only, Tarasov L.V. The book describes in an interesting and intelligible way…
• Visual Encyclopedia. All about the planet Earth and its inhabitants,. A detailed description of the history of the Earth from the Big Bang to the present day. Hundreds of color illustrations. Latest data, explanatory diagrams and drawings. Geochronological time scale. Wide view…

Geological scale

Geochronological scale - the geological time scale of the history of the Earth, used in geology and paleontology, a kind of calendar for time intervals of hundreds of thousands and millions of years.

According to modern generally accepted ideas, the age of the Earth is estimated at 4.5-5 billion years. In modern geology, the most common age estimate is 4.55-4.56 billion years, with an error estimate of several percent. Such estimates are based on the data of determining the age of rocks by radioisotope dating. The figure of 4.567 billion years is a kind of compromise between various datings of the age of rocks, which give figures from 4.2 to 4.6 billion years.

This time was divided into different time intervals according to the most important events that took place then.

The boundary between the Phanerozoic eras runs along the largest evolutionary events - global extinctions. The Paleozoic is separated from the Mesozoic by the largest Permian-Triassic extinction of species in the history of the Earth. The Mesozoic was separated from the Cenozoic by the Cretaceous-Paleogene extinction.

The history of the scale

In the second half of the 19th century, at the II-VIII sessions of the International Geological Congress (IGC) in 1881-1900. the hierarchy and nomenclature of most modern geochronological units were adopted. Subsequently, the International geochronological (stratigraphic) scale was constantly refined.

In geology, as in no other science, the sequence of establishing events, their chronology, based on the natural periodization of geological history, is important. Geological chronology, or geochronology, is based on finding out the geological history of the most well-studied regions, for example, in Central and Eastern Europe. Based on broad generalizations, comparison of the geological history of various regions of the Earth, patterns of evolution of the organic world at the end of the last century, at the first International Geological Congresses, the International Geochronological Scale was developed and adopted, reflecting the sequence of time divisions during which certain sediment complexes were formed, and the evolution of the organic world . Thus, the international geochronological scale is a natural periodization of the history of the Earth.

Among the geochronological divisions are distinguished: eon, era, period, epoch, century, time. Each geochronological subdivision corresponds to a set of deposits, identified in accordance with the change in the organic world and called stratigraphic: eonoteme, group, system, department, stage, zone. Therefore, the group is a stratigraphic unit, and the corresponding temporal geochronological unit is represented by an era. Therefore, there are two scales: geochronological and stratigraphic. We use the first when we talk about relative time in the history of the Earth, and the second when we deal with sediments, since in every place on the globe some geological events took place in any period of time. Another thing is that the accumulation of precipitation was not ubiquitous.

At present, there are three largest stratigraphic divisions - eonotems: Archean, Proterozoic and Phanerozoic, which correspond to zones of different duration in the geochronological scale. The Archean and Proterozoic eonotemes, covering almost 80% of the time of the Earth's existence, are distinguished in the Cryptozoic, since the skeletal fauna is completely absent in the Precambrian formations and the paleontological method is not applicable to their division. Therefore, the division of Precambrian formations is based primarily on general geological and radiometric data. The Phanerozoic eon covers only 570 million years, and the division of the corresponding eonoteme of deposits is based on a wide variety of numerous skeletal fauna. The Phanerozoic eonoteme is subdivided into three groups: Paleozoic, Mesozoic and Cenozoic, corresponding to major stages in the natural geological history of the Earth, the boundaries of which are marked by rather abrupt changes in the organic world.

The names of eonotems and groups come from the Greek words: "archeos" - the most ancient, most ancient; "proteros" - primary; "paleos" - ancient; "mesos" - medium; "kainos" - new. The word "cryptos" means hidden, and "phanerozoic" means explicit, transparent, since the skeletal fauna appeared. The word "zoi" comes from "zoikos" - life. Therefore, "Cenozoic era" means the era of new life, and so on. Groups are subdivided into systems, the deposits of which were formed during one period and are characterized only by their characteristic families or genera of organisms, and if these are plants, then by genera and species. Systems have been isolated in different regions and at different times since 1822. At present, 12 systems are distinguished, the names of most of which come from the places where they were first described. For example, the Jurassic system from the Jura Mountains in Switzerland, the Permian - from the Perm province in Russia, the Cretaceous - according to the most characteristic rocks - white writing chalk, etc. The Quaternary system is often called Anthropogenic, since it is in this age interval that a person appears. The systems are subdivided into two or three divisions, which correspond to the early, middle, and late eras. The departments, in turn, are divided into tiers, which are characterized by the presence of certain genera and species of fossil fauna. And, finally, the stages are subdivided into zones, which are the most fractional part of the international stratigraphic scale, which corresponds to time in the geochronological scale. The names of the stages are usually given according to the geographical names of the regions where this stage was distinguished; for example, Albanian, Bashkirian, Maastrichtian, etc. At the same time, the zone is designated by the most characteristic type of fossil fauna. The zone covers, as a rule, only a certain part of the region and is developed over a smaller area than the deposits of the stage.

All subdivisions of the stratigraphic scale correspond to the geological sections in which these subdivisions were first identified. Therefore, such sections are standard, typical, and are called stratotypes, which contain only their own complex of organic remains, which determines the stratigraphic volume of a given stratotype.

The specific names of the periods were given according to various criteria. The most commonly used place names. So, the name of the Cambrian period comes from lat. Cambria - the name of Wales when it was part of the Roman Empire, Devonian - from the county of Devonshire in England, Permian - from the city of Perm, Jurassic - from the Yuram Mountains in Europe. In honor of the ancient tribes, the Vendian (Wends - the German name for the Slavic people of the Lusatian Serbs), Ordovician and Silurian (tribes of the Celts Ordomviks and Silures) periods are named. Names associated with the composition of the rocks were used less frequently. The Carboniferous period is named because of the large number of coal seams, and the Cretaceous because of the widespread use of writing chalk.

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