about 1,000 metres (3,280 feet) thick, the coal-bearing series of the Moscow Basin about 250 metre (820 feet), and that of the Donetz Basin, in which coal also occurs, more than 1,000 metres (3,280 feet) thick. Marine and nonmarine deposits alternate rhythmically in these coal- bearing strata. Even over short horizontal distances, the clastic facies vary greatly in thickness. Because these deposits repre- sent debris of local origin they can vary greatly over short distances. In central Europe such deposits are up to 2,000 metres (6,560 feet) thick (Czechoslovakia, Ger- many); when a less abundant supply of sediment is avail- able, an entire series of strata miiy be as thin as 500 metres (1,640 feet). Intrusive and extrusive igneous rocks are of local to re- gional importance in the Lower Carboniferous. They oc- cur in the geosynclines, on the shelf (Kazakhstan), and in nonmarine areas (Scottish Midland Valley, Eastern Si- beria). Metamorphic rocks from the Dinantian are known prin- cipally from the high mountains that originated from the great Cordilleran Geosyncline and from the Eurasian Tethys Geosyncline in Mesozoic or Cenozoic time. Knowledge of these rocks is limited, as they are altered by the high pressures and temperatures of folding and mountain formation. The formation of new minerals and mechanical deformation of the rocks have often altered beyond recognition or destroyed the fossils that would have aided the determination of the age of the rocks. LOWER CARBONIFEROUS LIFE The Lower Carboniferous is characterized principally, and especially as compared to that of all older geological formations, by the abundance and variety of its plants. These plants are excellently preserved in the coal-bearing strata. During the course of the Devonian, plants had in- deed spread over large areas of the land surface (before this there had been only water plants); but in the Lower Carboniferous, because of particularly favourable cli- matic conditions, plant coverage was very dense. Trees appeared for the first time, grew to considerable heights, and developed woody trunks. The Psilophytales, which had constituted a considerable portion of the pre-Carboniferous flora, died out in the Carboniferous. The "coal forests" of the Lower Carbon- iferous consisted of Lycopodiales, lower vascular plants whose trunks reached heights of 30 metres. High-grow- ing trees with woody trunks also were found among the Articulata, but these also constituted the 4 6reeds" in the vicinity of the coal bogs and in moist locations, including coastal areas. For this reason Articulata are also known from marine deposits, particularly of the detritus facies, although less well preserved. A very notable advance in terms of evolutionary history was the appearance of the first seed plants (Pterido- spermales or Cicadoficiles). Gymnospermae also ap- peared for the first time, as trees. Among the algae (thallophytes), only those that precipi- tate lime are of geological significance. Sea algae existed in great quantity, sometimes forming reefs, and these be- came part of the limestone facies of the shelf regions. In the animal world, the great changes were afoot that led to the fauna of Mesozoic time. In the Carboniferous, animal life followed the plants onto the land. Crossop- terygii-like amphibians appeared, inhabiting the swamp- forests; some were more than five metres long. Of great importance for the history of life on Earth was the first appearance of the Goniatitidae (belonging to the Mollusca) at the lower border of the Carboniferous: these spiral-shelled animals, related to the cuttlefish of the present day, are among the decisive guide fossils for the determination of the age of strata from the beginning of the Carboniferous to the end of the Mesozoic. In the rest of the animal world no fundamental changes were to be observed. Because of the wide distribution of favourable conditions for Lower Carboniferous life (in the shelf seas large areas were covered with shallow wa- ter rich in nutrients and oxygen), a number of animal groups expanded into a great multiplicity of forms. mong the single-celled@animals this was true of the For- minifera; among the Echinodermata, it applied to the rinoids and blastoids, which appear particularly on the orth American midcontinent shelf as rock-forming . Among the mollusks, snails and mussels were chly developed. Surprisingly, the Brachiopoda declined n number of forms; this group inhabited the same envi- nments as the Mollusca. Whereas they had been pres- in extraordinary variety in the pre-Carboniferous sys- ms, they were represented by far fewer forms in the arboniferous. Only the small systematic group of the roductoidea expanded in the Carboniferous; almost all ther groups were impoverished. Not until the middle of e Mesozoic did the Brachiopoda again experience omething of a heyday before contracting again to the w species that survive today- In the series containing coal beds in the northeastern nited States, in Scotland, and in the European part of e Soviet Union, brackish water and lacustrine (lake- welling) organisms were rather widely distributed and merous for the first time in the Earth's history. The a consisted predominantly of mussels, which resem- led in structure the brackish-water mussels of today. The marine fauna known from the Lower Carbonifer- us constituted a very uniform group. The occurrences n what are now separate continents indicate that the more or less contemporaneous fauna often consists of dissimilar species; endemic species also occur. But at the same time a considerable number of genera are found throughout the world. There were as yet no faunal or floral provinces, which first began to develop in the Up- per Carboniferous and which are so clearly delineated today. This uniformity is the more astonishing because similar fauna inhabited regions of dissimilar climate. The Lower Carboniferous formations of North Amer- ica, Europe (including Spitsbergen), and North Africa originated in the vicinity of the Equator, in tropical and subtropical climates. In these regions, coal beds are found that were formed in tropical or subtropical swamp for- ests and bogs during Carboniferous time. On the other hand, the similar marine fauna of eastern Australia must have developed in a significantly colder climate, for in the Upper Carboniferous there appear throughout the southern part of Australia traces of glaciation; this is also true in South Africa and South America. The fauna contained in limestones, which formed in the Mg shelf regions at the periphery of the continental shields ver or over substrata consolidated in pre-carboniferous time, far consist mainly of benthonic organisms-those living on the ocean floor. Particularly richly developed in these facies are the calcareous algae, which also formed reefs: the Stromatopora, Tabulata, Coralla, and Bryozoa. A va- riety of forms developed in the Foraminifera, in the productid brachiopods and in the snails and mussels. The trilobites of these facies have not yet been thoroughly studied; the Phillipsiidae and Brachymetopus seem to predominate. Among the Echinodermata, the crinoids and blastoids of the North American midcontinent shelf are well known for their variety and good preservation. The much poorer fauna of the predominantly clastic fa- cies, which developed in the deeper seas of the orogenic regions, consisted mainly of swimming and floating types of life. The seawater at greater depths was so poorly sup- plied with oxygen that for the most part only a few spe- cialized species, such as primitive burrowing animals, could live there. With the exception of these, the ocean floor was little inhabited. Cephalopods, relatives of the present-day cuttlefish, are particularly characteristic of these facies, which are also called Culm Facies. Mussels that lived attached to floating seaweed masses, swimming trilobites, and conodonts (q.v.) were numerous and wide- ly distributed. The two types of fauna, the benthonic of the shelf seas and the nektonic-planktonic of the deeper basins, hardly ever occurred together in the same region. The bounda- ries between shelf and basin are extraordinarily sharp. The compositions of the rocks and of the fauna change almost in a hairbreadth and as far as is known scarcely ever blend. Because the ages of rocks are determined with Encyclopedia Britannica