448 



THE AGE OF MAMMALS 



Epochs 



6. Present time 

 Holocene 



5. Post-Lahontan 

 (?)post-Glacial 



4. Second rise of Lake La- 



hontan 



(?) Period of final gla- 



ciation. 

 3. Inter-Lahontan Period 



Interglacial 



2. First rise of Lake Lahon- 

 tan 

 (?) First glacial advances 



1. Pre-Lahontan 

 (?)Pre-Glacial 



Climate 



Infrequent rain, evaporation 

 rapid, dry climate, lacus- 

 trine bodies small and 

 varying, no glaciers. 



Arid period. Great dryness, 

 average temperature higher 

 than now. Lakes dried 

 up, glaciers melted. 



More rain than in period 2. 

 Slight evaporation, low 

 temperature, large lakes, 

 and powerful glaciers. 



Less moisture, more evapora- 

 tion, higher temperature. 

 Lakes smaller than at 

 present. Disappearance 

 of glaciers. 



Low temperature, abundant 

 moisture, little evapora- 

 tion, glaciers on the moun- 

 tains large, lakes in the 

 valleys. 



Dry period, little rain, much 

 evaporation, lakes small, 

 often dried up, mountains 

 without glaciers. 



Mammals 



Obsidian spearhead. 

 Elephas columbi, 

 Equus, Bison, 

 {f)Eschatius. 



These periods of moisture are attributed to the southward and north- 

 ward movements of the rain-belt, along the advancing and retreating front 

 of the ice-belt. 



At its maximum, Lake Bonneville covered an area of 19,000 square 

 miles, and was 1,000 feet deep, whereas its diminutive descendant, the Great 

 Salt Lake, is less than fifty feet deep. Lake Lahontan covered an area of 

 nearly 9,000 square miles. All lines of physical or erosive evidence point 

 to the shortness of time since the last rise of Lakes Bonneville and Lahontan. 



In both basins, during the period of the second rise, we have glimpses both 

 of the mammalian life (p. 468) and of the bird life (p. 460), so that it is of 

 the utmost importance and interest to correlate this 'second rise' in time 

 with one of the five epochs of great glacial advance in the central states of 

 Iowa and Illinois. This can only be done through a closer comparison of 

 the specific forms of Pleistocene mammals than has been attained at present. 



Migrations of Sangamon flora in Canada. — Deposits on the northern 

 shores of Lake Ontario afford significant evidence of alternate warmer and 

 colder periods respectively in the Don and overlying Scarborough forma- 

 tions. As in Europe, there are proofs that certain interglacial epochs were 



