S6 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. 



the period, and the substitution of new and more dominant 

 forms. 



The cHmatic conditions resulting from the changes in the 

 physical geography, permitted, or caused, the migration of ani- 

 mals and plants. 



LactT migrations were also forced by subsequent changes 

 and the resulting environments, and were further made 

 possible by the land connections between continents and con- 

 tiguous islands, (previously separated by bodies of water) by 

 the elevation of the land. 



This elevation, connected possibly with astronomical and 

 other causes not thoroughly understood, brought about the Gla- 

 cial Period, which may be termed the last critical period which 

 has occurred. 



This was also attended by well-marked oscillations of the 

 earth's crust by elevations and depressions, and especially is this 

 apparent in high latitudes where the immense areas of the ice 

 bodies, gradually flowing towards the equator, planed down the 

 most accentuated irregularities of the surface caused by the up- 

 heavals. 



These ice bodies picked up large bodies of projections upon 

 which they were formed, or with which they came in contact, 

 and transported them to long distances, crushing and grinding 

 them up on the way by their irresistible power, thus forming 

 boulders, gravel, sand, and soil, which, as the ice approached 

 the less elevated and warmer latitudes: and was gradually 

 melted, were deposited in the valleys and on the plateaus. 



It will be thus seen that, the glaciers were important fac- 

 tors in preparing the earth for its occupancy by man and the 

 higher animals, and as will be shown as we proceed, the ice 

 drove animal and vegetable organisms towards the equatorial 

 regions, causing an admixture of the faunas and floras of widely 

 separated regions. 



After the recession of the great ice fields and glaciers con- 

 sequent upon another depression, there came a period when our 

 coast enjoyed the advantages of a tropical climate, during which 

 time many genera of plants and animals inhabited the land and 

 probably continued until the epoch of volcanic disturbances, 

 when, instead of being buried under a sheet of ever-moving 

 ice, the country was overwhelmed by a cataclysmic flow of 

 molten lava, which doubtless destroyed all the animal and veg- 

 etable life of the region, filling up the river channels, and en- 

 gulfing the mountains of ordinary altitudes ; a time when 

 the higher peaks, instead of being islands surrounded by ice 

 fields, or water, were left projecting from an immense sea of 

 molten lava, which congealed and may be still seen spreading 

 over vast areas of land. ,,,, , .. ,, 



( lo be continued). 



