PAPERS ON GEOLOGY 275 
EXPLANATION OF PLATE II. 
Cyclocystoides ornatus n. sp. 
Fig. 1. View showing ring of submarginal plates of type specimen, 
X 1%. Orchard Creek Shale. 
Pterinea oblonga n. sp. 
Fig. 2. View of right valve of type specimen, natural size, Orchard 
Creek shale. 
Isotelus convexa n. sp. 
Fig. 4. View of cranidium lacking free cheeks. 
Fig. 5. Free cheek of a large individual. 
Fig. 6. Dorsal view of pygidium. 
Fig. 3. Hypostome thought to belong to this species. Natural size, 
Orchard Creek shale. 
THE CLIMATIC HISTORY OF ALASKA FROM A 
NEW VIEWPOINT 
Exiot BLACKWELDER, UNIVERSITY OF ILLINOIS 
In the earlier days of geologic study, inferences regarding 
the climates of bygone periods were drawn chiefly from fossils. 
Palm leaves in Greenland and musk-ox bones in Kentucky gave 
their obvious testimony. Even in those days, however, the 
climatic significance of certain kinds of sediments was well 
recognized. But at first, it was only the peculiar and extreme 
sedimentary types, such as glacial till and beds of rock salt or 
gypsum, that were interpreted in this manner. Ordinary sand- 
stones, shales and conglomerates were hardly suspected of con- 
taining a hidden record of ancient climates to which the key 
had not yet been found. 
Within the last few years, however, we have come to un- 
derstand that the formation of sedimentary rocks is controlled 
by several factors and that one of the most important—in 
many cases the dominant—factor is climate. As the influence 
of the climatic factor comes to be understood, we find ourselves 
able, with increasing confidence, to read from the sediments the 
climatic conditions under which they originated. In this re- 
spect we owe much to Professor Joseph Barrell of Yale. 
At present the climatic significance of the continental sedi- 
ments is much better understood than that of the marine de- 
posits. There is however, good reason to hope that we may 
