GEOLOGY 



2440 



GEOLOGY 



highest class of mollusks. Some of these grew 

 to immense size, bearing shells twelve or fif- 

 teen feet long and a foot in diameter. On the 

 land there were flying insects, implying the 

 existence of vegetation and of an atmosphere 

 that would be adapted to air-breathing crea- 

 tures. The actual record of land plants, how- 

 ever, is meager and unsatisfactory. 



The Silurian Period is marked by the for- 

 mation of coral reefs, the first appearance of 

 scorpions, and a rich and varied development 

 of echinoderms (which see). Fossils of land 

 life continue to be meager, and likewise little 

 is known of plant life of the sea. Following 

 this period is the Devonian, some of the rock 

 formations of which are of importance com- 

 mercially. The Upper Devonian, for example, 

 is the chief source of oil and gas in the western 

 section of Pennsylvania and one of the sources 

 in West Virginia; the Middle Devonian in 

 Ontario produces oil; and the Old Red Sand- 

 stone of Great Britain and Ireland is of the 

 same period. In certain sections of Germany 

 large quantities of iron, tin and copper have 

 been taken from the Devonian series. 



During this period (and thereafter) marine 

 fishes were abundant, and there were sharks 

 having fin-spines a foot in length. Barnacles 

 of the modern sessile type, that is, attached 

 directly by the base to other objects, made 

 their first appearance, and there appeared and 

 declined in this division of time a strange ani- 

 mal called ostracoderm, which formed a link 

 between arthropods and vertebrates. It was 

 related to the fishes, but entirely lacking in 

 vertebrae. Plants, snails, insects, myriapods 

 (thousand-legged worms), scorpions and am- 

 phibians (which see) are known to have lived 

 on the land, and the Devonian Period saw the 

 origin of ferns and of the gigantic progenitors 

 of horsetail rushes and club mosses (see HORSE- 

 TAIL RUSH). Fernlike plants predominated. 



The Carboniferous Period, divided into 

 Lower and Upper Carboniferous, is an espe- 

 cially important epoch in geologic history from 

 an economic standpoint. To the second divi- 

 sion may be referred the rich coal measures 

 of Pennsylvania (anthracite and bituminous), 

 coal sections in Michigan, Illinois, and the 

 states from Iowa to Texas, and the coal de- 

 posits of Nova Scotia and New Brunswick 

 (see COAL). Iron ores of the system occur 

 in Pennsylvania and Eastern Ohio, and oil and 

 gas in Oklahoma, Kansas and Illinois. In 

 Europe, workable coal of the Upper Carbonif- 

 erous system is found in Great Britain, Ireland, 



Belgium, France, Spain, Germany, Austria and 

 Russia, and the system is coal-producing else- 

 where, as in China and Brazil. 



From the Lower Carboniferous stage comes 

 the earliest wood which shows rings, but the 

 record of land life is as a whole inadequate. 

 Sharks were supreme in the open seas and were 

 more abundant than in any later period. On 

 the other hand the record of land life of the 

 Upper Carboniferous stage is unusually full. 

 Gymnosperms (plants whose seeds are not en- 

 closed in a seed-case) were present in great 

 abundance. Giant ancestors of the modern 

 horsetails grew in the forests to a height of 

 sixty to ninety feet, and there were abundant 

 growths of huge club mosses and fernlike 

 plants. Since coal is of vegetable origin, the 

 coal measures bear wonderful records of the 

 complex plant life of the Upper Carboniferous 

 age. 



Animal life in this stage of the world's 

 history is also abundant. On the land lived 

 amphibians, insects (including cockroaches, lo- 

 custs, crickets and bugs, but no moths, but- 

 terflies or flies), spiders, scorpions, myriapods 

 and land snails. It is in the later coal meas- 

 ures that fossils of amphibians (the first land 

 vertebrates) are first found in abundance and 

 variety. Another important feature of the age 

 was the development of fresh-water fishes, mol- 

 lusks and crustaceans. 



In the final period of the Paleozoic Era, the 

 Permian Period, there were gigantic geographic 

 changes, through which great areas of sea bot- 

 tom were converted into land. As a result, 

 both plant and animal life became greatly 

 impoverished. Of the new plant types which 

 appeared one is thought to be the ancestor 

 of the group which includes the giant sequoia 

 (which see). Before the end of the period the 

 amphibians were overshadowed in numbers 

 by the reptiles, their probable descendants. 

 Though the differentiation between the two 

 groups began earlier, the reptiles did not ap- 

 pear as a large and complex division until 

 well into the Permian Period. Marine life was 

 greatly depleted, but the fresh waters teemed 

 with fishes of a somewhat modern character. 

 Geologists have found that the Permian age 

 presents numerous problems that are difficult 

 of solution. This is due to widespread glacia- 

 tion in India, Africa and Australia, near and 

 within the tropics. 



The Triassic Period, which ushered in the 

 Mesozoic Era, was characterized by an ex- 

 traordinary development of the reptilian class, 



