GEOLOGY 



2441 



GEOLOGY 



including crocodiles, flying saurians, lizards 

 and other scaled reptiles. Before the period 

 closed certain reptiles had made their home 

 in the sea. There also appeared the race of 

 dinosaurs (see DINOSAURIA), ungainly monsters 

 that attained full development later in the 

 era. In the Triassic series are fossils of a 

 primitive form of mammals. 



The Jurassic Period was characterized by an 

 extension of the seas, as a result of which ma- 

 rine life again assumed a leading place in the 

 geologic record. Among the fishes were the 

 forerunners of the modern garpikes and stur- 

 geons, and various sea reptiles made their ap- 

 pearance. Land life is especially interesting 

 in that it exhibits slow progress towards types 

 that exist to-day. The leading plants were 

 cycads, conifers, ferns and horsetails. Among 

 the animals were flesh- and herb-eating dino- 

 saurs, some of the latter reaching a length of 

 sixty feet. The herbivorous dinosaurs, which 

 are first known in the Jurassic system, were 

 horrible-appearing monsters, but unwieldy and 

 stupid. In this period flying reptiles reached 

 full development. Though small at this time, 

 they later attained a wingspread of nearly 

 twenty feet. 



Of especial importance is the differentiation 

 in this period of the oldest-known bird, a de- 

 scendant of the reptiles. It had reptilelike 

 claws, but its front limbs were adapted for 

 flying and it was clothed with feathers. Traces 

 of its reptilian ancestry may be seen in its 

 long vertebrated tail and its toothed jaws. Up 

 to this time the fossil record of mammals is 

 very meager. 



The Mesozoic Era closes with the Cretaceous 

 Period, divided into Lower and Upper Creta- 

 ceous. By some geologists the Lower Creta- 

 ceous is known as the Comanchean and the 

 two are considered as distinct periods. The 

 earlier of these is characterized by one of the 

 most striking evolutions in the history of 

 plants the appearance and widespread devel- 

 opment of angiosperms (which see), plants 

 whose seeds are enclosed in a seed-case. (It 

 is possible that they had their origin in the 

 late Jurassic Period.) With the development 

 of flowering plants there was probably a cor- 

 responding development of insect life. 



The later Cretaceous age is preeminently the 

 coal period of the western part of the North 

 American continent; and the close of the pe- 

 riod was marked by extraordinary volcanic 

 activity. Plant life on land began to assume 

 a modern aspect, and -among the present-day 



genera which appeared were those which in- 

 clude the birch, beech, oak, walnut, sycamore, 

 tulip-tree and maple. Grasses had their origin 

 in this age, and palms grew abundantly. The 

 most important plants that support animal life 

 had now made their appearance, laying the 

 foundation for the future marvelous evolution 

 of the higher animals. Reptiles, espepially 

 dinosaurs, continued their dominance among 

 land animals; birds existed both on land and 

 sea, but the marine birds were the more im- 

 portant. Turtles appeared for the first time 

 in the ocean, and some of these were of enor- 

 mous size. 



The final era, the Cenozoic, brings us up to 

 the earth as we know it to-day. The transi- 

 tion of life from the Mesozoic to the Cenozoic 

 was characterized by four significant features: 

 the appearance of new marine species; the 

 practical disappearance of the great saurians 

 and decided changes in other reptiles; the pre- 

 dominance of mammals, and the continuance of 

 large numbers of Mesozoic land plants; 

 Through the different periods of the Cenozoic 

 there was steady progress toward life as it now 

 exists. Before the close of the Eocene Period 

 nearly all existing groups of mammals had 

 become well defined. Primitive types of the 

 horse, pig, hippopotamus, camel, dog and opos- 

 sum appeared, and some mammals, like the 

 land reptiles of the preceding era, took to the 

 sea. Thus originated whales, dolphins, por- 

 poises, manatees7"dugongs, seals and sea lions. 

 Nearly*all types of marine invertebrates had 

 assumed their, modern forms. 



The evolution of the higher animals through 

 succeeding periods need not be discussed in 

 detail. Representatives of primates, the high- 

 est order of mammals, to which man himself 

 belongs, appeared in the Miocene Period, but 

 the geologic record is singularly meager in re- 

 gard to the origin of the human race. When 

 geologists have had opportunity to study more 

 carefully the tropical regions of the Eastern 

 Hemisphere, the original home of the race, we 

 may know more about this fascinating subject. 

 It is also an unanswered question as to whether 

 or not man existed in North America during 

 the Pleistocene Period (see GLACIAL PERIOD), 

 when thousands of square miles of its surface 

 were covered by thick ice sheets. This and 

 many other questions must be left to future 

 investigations and discoveries. The forces that 

 have brought the earth to its present condition 

 have worked through unknown millions of 

 years, and changes are still going on as the 



