594 ROLLIN D. SALISBURY 
cance, each ice-sheet meant the relatively rapid superposition upon the 
northern continents, over the great areas indicated, of a new layer of 
rock, the ice, which for tracts of millions of square miles must have 
had a thickness exceeding a thousand feet, and perhaps a thickness of 
several thousand feet. The aggregate volume of this new rock, 
superposed on the northern parts of the northern continents, was such 
that it could only have been measured in terms of millions of cubic 
miles. The withdrawal of its substance from the sea effected a cor- 
responding lowering of its surface, an appropriate extension of land, 
and an increase in its height above the sea. : 
Though. this great body of rock new-laid upon the lands was 
temporary in its character, primarily because of the low temperature 
at which its substance assumed the liquid form, it was of great 
importance, from a geologic point of view, in more ways than one. 
t. In the first place, the loading of millions of square miles of land 
with such a weight must have had an appreciable effect upon crustal 
movements, if the doctrine of isostasy has validity, and its disappear- 
ance, under climatic conditions which developed later, must have 
produced movements of the same class, but of opposite phase. 
2. Again, the development of the ice-sheets put a virtual stop to the | 
processes which had been in operation over six millions of square 
miles of the land, and set other processes into operation in the same 
places. ‘Thus the normal phases of river work were suspended, most 
rivers within the ice-covered area ceasing to flow altogether. The 
usual phases of rock weathering and decay were practically stopped 
over the same areas, areas which, in the aggregate, were a very con- 
siderable fraction of the surface of the land. On the other hand, a 
new process of erosion was substituted for the old—erosion not 
restricted chiefly to the removal of decayed rock. 
3. The changes in erosion were hardly greater than those in 
sedimentation, for instead of the assortment and separation of 
decayed material into its several physical classes before deposition, 
fine sediments and coarse, largely of undecayed material, were left 
promiscuously commingled. ‘Thus on a large scale and over enor- 
mous areas deposits were made which were unlike those of comparable 
extent at any other stage of the earth’s history, unless at times when. 
climates were similar. 
