THE AGE OF ICE. 651 



Erratics. — These are of all shapes and sizes, occasionally reaching 

 colossal proportions, and containing many hundred feet. Some are 

 rounded, others are angular, and not a few exhibit marks of scarifica- 

 tion. They may rest on base-rock, and, if carefully poised, may be 

 made to oscillate by the form of the land, or these large blocks may 

 appear on the till, angular debris, and hills of gravel. As a general 

 rule, they prove to have been carried from higher to lower levels in 

 Scotland, though many exceptions are recorded. There is one at the 

 height of 1,020 feet on the Pentland Hills, which may have traveled 

 westerly as much as eighty miles. It probably passed from one moun- 

 tain across a wide valley before attaining its final resting-place. This 

 is not so striking as the blocks lying nearly over the recently-com- 

 pleted Hoosic Tunnel, in Western Massachusetts, one of which weighs 

 510 tons, and has been transported fromOakHill across a valley 1,300 

 feet deep. It has hundreds of lusty comrades, scattered in a south- 

 easterly course for thirty miles. 



Sometimes a large block is revealed by the washing away of the 

 till around it. Those on the surface of gravel may have been carried 

 by floating ice. To such blocks it is not easy to assign limits of the 

 distance traveled, since icebergs may float for thousands of miles with- 

 out melting. 



Origin of the Cold Climate. — The question of the cause of the 

 glacial cold has been discussed warmly for a long time. The opinion 

 seems to be gaining ground that purely geological causes are not 

 sufficient to account for the magnitude of the glacial distribution. The 

 precession of the equinoxes, changing the times of the seasons, and the 

 eccentricity of the earth's path around the sun, lengthening the win- 

 ters and increasing precipitation of moisture, when combined with 

 certain changes in the courses of ocean-currents, and some elevation 

 of land in the north, may have together been instrumental in bringing 

 around a period of intense cold. If it be possible to use the orbital 

 changes as a guide to a chronological date for this term of cold, we 

 can say it began about 240,000 years since, and continued for 160,000 

 years, terminating 80,000 years before a. d. 1260. The cold would 

 have culminated about 30,000 years after its beginning. 



Granting such figures, we can understand that the glacial must 

 have been the dark age in the earth's history — a terrible blight upon 

 the flourishing faunas and floras existing in tertiary times in northern 

 latitudes. The presence of warm temperate plants in Greenland has 

 always excited interest, even to the proposal of very wild theories to 

 account for the genial climate there of preglacial days. It may be 

 that the American Sequoia traveled across the bridge anciently con- 

 necting Greenland with Iceland and Scotland, and that the renowned 

 cedars of Lebanon are the cousins of their famed relations in Cali- 

 fornia ; but the connection has been severed by the ruthless ice-flow, 

 and is not likely to be reestablished, unless our sun shall carry his sys- 



