This theory, also known as descent with modification, constitutes organic evolution...... A relative—and, subsequently, absolute—chronology of the Stone Age was established on the basis of strati-graphic data. The first two terms quickly lost their importance, but the third, which was proposed by the Italian geologist G. This approach is based on the radioactive decay of various elements—such as potassium, uranium, and lead—contained in the minerals of sedimentary and magmatic rocks (see).Stratigraphy is especially important in the study of settlements with a thick and, sometimes, greatly disturbed cultural layer—for example, primitive settlements, cities of the ancient East, Greek and Roman cities, and medieval cities. Arduino in 1759, is still used by non-Soviet geologists. Information on the radiometric age of sedimentary rocks is rather meager.In such cases, each layer reflects a certain stage in the settlement’s or city’s history. In 1829, the term “Quaternary” was proposed by the French geologist J. The potassium-argon age method makes use of very rare potassium salts (carnallite) and glauconite, which is common in sedimentary rocks.

Stratigraphy is based primarily on the capacity to trace strata of sedimentary rocks and on the investigation of faciès changes of the rocks in basins of past geologic epochs. In the first half of the 19th century, as a result of research carried out in various countries, stratigraphic systems were identified, and their sequence in the history of the earth was established. The uranium-thorium-lead age method is used to date zircons from effusive rocks.

The fossil content of sedimentary strata reflects the irreversible development of organic life and plays a fundamental role in establishing that deposits being studied are of the same age (see). At the session of the International Geological Congress held in Bologna in 1881, a standard classification of the subdivisions of the geologic history of the earth was adopted, and the previously identified systems and series were given official acceptance. Reneviez proposed a composite “chronograph”; it included not only the systems and series but also stages, which were used with a number of changes and refinements. Considerably more complete data on the age of rocks can be obtained by the indicated methods for various intrusive rocks injected into sedimentary strata.

For this reason, stratigraphy is closely connected with paleontology and with geochronology, which studies the age and the chronological sequence of formation of the rocks making up the earth’s crust. At the turn of the 19th century, the British engineer W. A great contribution to the creation and development of a geostratigraphic scale was made by the Russian and Soviet geologists A. The basic difficulty here consists in tying these precise figures to the geologic column; for this purpose, the contacts of the intrusive body with the sedimentary strata are carefully studied.

Statigraphy is therefore important in the relative dating of all types of rock. Such study is essential in the relative dating of the layers, structures, burials, and artifacts and is of particular importance in cases where the natural order of the layers has been disturbed by, for example, digging, cave-ins, landslides, or erosion. Traces of the vital activity of blue-green algae are encountered in very large quantities.

In areas where the strata have undergone folding, faulting, and erosion, stratigraphic techniques are used to determine their correct sequence. Spencer, Basic Concepts of Historical Geology (1962); R. The stratigraphy of archaeological remains is established by studying vertical sections. In the 1960’s these algae came into use for the differentiation and correlation of carbonate strata of the Upper Precambrian.

The principle of included fragments in stratigraphy states that any rock fragment included in another rock must be older than the surrounding rock. By means of typology and other archaeological methods, as well as methods of the natural sciences, a transition can be made from relative stratigraphic dating to absolute dating (see). The paleontological method has not yet been applied to older deposits.

Fossils have been the most important means of correlation because, as a result of evolutionevolution,concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. Stratigraphic conclusions based on a single remain can often be used to establish the relative dating of remains in an entire area. In the late 18th century, the constituent deposits of the earth’s crust in Western Europe were divided into Primary (now Paleozoic), Secondary (Mesozoic), and Tertiary (Cenozoic) formations. Radiometrie dating is acquiring primary importance for older deposits.

stratification(Lat.,=made in layers), layered structure formed by the deposition of sedimentary rocks. Younger geological units overlie, embay, or intrude older units. Even small pieces of sedimentary rocks contain hundreds or thousands of such organisms, a fact of particular importance in determining the age of rocks in cores from boreholes.

Changes between strata are interpreted as the result of fluctuations in the intensity and persistence of the depositional agent, e.g...... Stratigraphy is based on the law of superposition, which states that in a normal sequence of rock layers the youngest is on top and the oldest on the bottom. It is therefore possible to produce geological maps of the surface of a planet or satellite, and to form a stratigraphic classification to help to understand its history. Special mention should be made of spore and pollen analysis, which is used for determining the age of sedimentary strata in all subdivisions of the Phanerozoic.