276 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



We have also seen that the Proterozoic closed with a frigid climate, as is attested by 

 the tillites of Australia, Tasmania, and possibly China, while the other glacial deposits 

 of India, Africa, Norway, and Keweenaw certainly do in part indicate another and older 

 period of cool to cold world climates. 



Cambric. — Due to the researches of many paleontologists, but mainly to those of 

 Charles D. Walcott, we now know that the shallow-water seas of Lower Cambric time 

 abounded in a varied animal life that was fairly uniform the world over in its faunal develop- 

 ment. It was essentially a world of medusaj, annelids, trilobites, and brachiopods, animals 

 either devoid of skeletons or having thin and nitrogenous external skeletons with a limited 

 amount of lime salts. The "lime habit" came in dominantly much later, in fact, not 

 before the Upper Cambric. However, that the seas in Lower Cambric tune had an abun- 

 dance of usable lime salts in solution is attested by the presence of many Hyolithes, small 

 gastropods and brachiopods, and more especially by the great number of .Ai-chsocyathina!, 

 which made reefs and hmestones 200 feet thick and of wide distribution in Australia, 

 Antarctica, California (thick limestones near the base of the Waucoba section), southern 

 Labrador (reefs 50 feet thick), and to a smaller extent in Nevada, New York, Spain, 

 Sardinia, northern Scotland, and Arctic Siberia. 



With an abundance of limestone and reef-making animals of world-wide distribution 

 in the Lower Cambric, we must conclude that the climate at that time was at least warm and 

 fairly uniform in temperature the world over. We therefore see the force of a statement 

 made to the writer by Walcott some years ago, in a letter, that "the Lower Cambrian fauna 

 and sediments were those of a relatively mild chmate uninfluenced by any considerable 

 extent of glacial conditions," and also that "the glacial chmate of late Proterozoic time 

 had vanished before the appearance of earliest Cambrian time." 



Toward the close of Lower Cambric time there was considerable mountain-making, 

 without apparent volcanic activity, going on all along eastern North America and to a 

 lesser extent in western Europe. These uplifts seemingly had much effect upon the marine 

 hfe, for the Middle Cambric faunas became more and more provincial in character in 

 comparison with the earlier, more cosmopohtau faunas of Lower Cambric time. 



The Archseocyathinfe, which had endured since earUest Proterozoic time, now van- 

 ished, and their extinction is suggestive of cooler waters; there was, however, a greater 

 variety of invertebrate forms, more lime-secreting invertebrates, and far more widespread 

 limestone deposition in Middle Cambric time. In the LTpper Cambric the brachiopods, 

 gastropods, cephalopods, and bivalve crustaceans were abundantly represented by thick- 

 shelled forms, and in most places throughout North America there was marked deposition 

 of limestones, magnesian hmestones, and dolomites, all of which is suggestive of warmer 

 waters. 



Ordoiicic a7id Siluric. — The Ordovicic seas from Texas far into the Ai-ctic regions were 

 dominated by limestone deposits and a great profusion of marine life that was also more 

 highly varied than that of any earlier time. The same species of graptohtes, brachiopods, 

 bryozoans, trilobites, and other invertebrate classes had a very wide distribution, all of 

 which is evidence that at that time the earth had mild and uniform climates. In the 

 Middle Ordovicic and again late in that period reef corals were common from Alaska to 

 Oklahoma and Texas (Vaughan, 1911). 



Toward the close of the Ordovicic, mountain-making was again in progress throughout 

 eastern North America without significant volcanic activity, but in western Europe, 

 where the movements were less marked, volcanoes were more plentiful. The seas were 

 then almost completely withdrawn from the continents, and yet when the Siluric waters 

 again transgressed the lands we find not only the same great profusion and vaiiety of hfe 

 as before, but as widely extended limestone deposition. The evidence is again that of mild 

 and uniform chmates. We can therefore say that the temperatures of air and water had 



