306. ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
that there were at least two and probably three other periods of 
widespread glacial climates. All of these were geologically very 
ancient, earlier than the Paleozoic; in fact, one was at or near the 
close of Proterozoic time, while another was at the very beginning of 
that era and almost at the beginning of earth history as known to 
geologists. 
The oldest of all glacial materials occurs at the base of the Lower 
Huronian and is of great extent in Canada. Seemingly of the same 
time is the Torridonian glacial testimony of northwest Scotland. 
The Proterozoic tillites of China in latitude 31° N. may also be of 
this time. If these correlations are correct, then the oldest glacial 
evidence indicates that a greatly cooled climate prevailed near the 
very beginning of the known geologic record and that it was dominant 
in the Northern Hemisphere. 
Toward or at the close of the Proterozoic there is other evidence 
of a glacial climate in Australia, Tasmania, and Norway. These 
occurrences of tillites lie immediately beneath Lower Cambric 
fossiliferous marine strata and probably are of pre-Cambric age. 
In India there is also evidence of late Proterozoic tillites in two 
widely separated places, and it may be that the inadequately studied 
Keweenawan testimony of the Lake Superior region is of this time. 
If so, these occurrences record a distribution of glacial materials very 
similar to that of Permic time. Again, the Proterozoic tillites of 
Africa are clearly of another age, so that there is evidence of at least 
three periods of glaciation previous to the Paleozoic. 
The physical evidence of former glacial climates is even yet not 
exhausted, for the Table Mountain tillites of South Africa point to a 
cold climate that apparently occurred, at least locally, late in Siluric 
time. Finally, ghere may have been a seventh cool period in early 
Jurassic time (Lias), but the biologic evidence so far at hand indicates 
that it was the least significant among the seven probable cool to 
cold climates so far discovered in the geologic record. 
The data at hand show that the earth since the beginning of 
geologic history has periodically undergone more or less widespread 
glaciation and that the cold climates have been of short geologic 
duration. So far as known, there were seven periods of decided tem- 
perature changes, and of these at least four were glacial climates. 
The greatest intensity of these reduced temperatures varied between 
the hemispheres, for in earliest Proterozoic and Pleistocene time it lay 
in the northern, while in late Proterozoic and Permic time it was 
more equatorial than boreal. The three other probable periods 
of cooled climates are as yet too little known to make out their 
centers of greatest intensity. 
Of the four more or less well-determined glacial periods, at least 
three (the earliest Proterozoic, Permic, and Pleistocene) occurred 
during or directly after times of intensive mountain making, while 
