C. Schiicliert— Evolution of Geologic Climates. 323 



tion of species and genera is beginning, along with the in- 

 troduction of biotic lines that are to lead to the terminal 

 giants. It is true that in these stress-faunas some of the 

 handicaps of the environment are due to physical causes 

 other than temperature, and yet the chief deterrent seem- 

 ingly was the lack of the proper warmth. What kills off 

 the giants ? In some cases it is old age within the stock, 

 but in many others it apparently is change in the environ- 

 ment, the deciding factor in which appears to be temper- 

 ature. 



The evaluation of sediments for climatic purposes has 

 not yet gone very far, nevertheless it is well known that 

 limestones are far more readily deposited by the life of 

 the warm waters than by those of cold ones. The great 

 limestone-making areas of to-day are in the warm waters. 

 Moreover it ought to be well known that the greatest 

 amount of limestones, and especially of magnesian lime- 

 stones and dolomites, does not occur in polar lands but 

 rather in the temperate and subtropical ones. At times 

 they are of extremely wide geographic distribution, as are 

 the magnesian limestones and dolomites of the Ozarkian 

 and early Ordovician, and those of the middle Silurian, 

 middle Devonian, and middle and late Pennsylvanian — 

 times when the earth's climates are recognized by general 

 agreement as mild throughout most of the northern hem- 

 isphere. AVhy is it that pure limestones and dolomites 

 are less prevalent at other times during the Paleozoic and 

 even within the latitude of the United States? In some 

 cases it is undoubtedly because of the too great preva- 

 lence of muds and highlands, but in others it appears to 

 be due to a temperature condition. 



In 1918, Blackwelder published a short but very striking- 

 paper entitled ^^The climatic history of Alaska from a 

 new viewpoint. ' '^ The view^point is that of the sediments, 

 and one is impressed w^ith the abundance he records of 

 sombre and dark colors, muddy sandstones, and silts, 

 and undecomposed mineral elastics, the scarcity of lime- 

 stones, and the almost total absence of red colors. From 

 the evidence -recited Blackwelder concludes: 



' ' The combined evidence strongly suggests that the cool, moist 

 climate of modern Alaska, — oscillating now and then toward the 

 glacial Arctic condition on the one hand and toward the moist 

 temperate on the other, — has been persistent, with but few real 

 interruptions, throughout the known geologic history of Alaska. ' ' 



* Eliot Blackwelder, Trans. Illinois Acad. Sci., 10, 275-280. 



