No. 5.] SPENCER — SURFACE GEOLOGY. 285 



ward, where there were any elevated peaks or table lands. These 

 highlands were coostantly sending off icebergs which, breaking 

 loose, were borne southward by the oceanic or lacustrine currents, 

 and carrying with them their loads of stones and debris from the 

 region of their foundation. The striations of the rock surfaces 

 in cootioental areas, remote from glacial-producing mountains 

 (or hills perhaps) was accomplished by the stranding of the bergs 

 in the comparatively shallow basins. This action is shown to-day 

 on the coast of Labrador and Greenland. At the same time the 

 melting bergs were depositing their loads as boulder clay. The 

 iceberg theory accounts for the boulder clay of the St. Law- 

 rence and the stratified Erie clay (with boulders) of the lake 

 region, both dating back not only to Dana's Champlain epoch, 

 but also to the epoch of his Glacial Drift. 



There is no material difference in the explanations of the origin 

 of the middle and later deposits of the Glacial period, as rendered 

 by the more liberal view of the glacial and iceberg hypotheses, 

 both recognizing the subaqueous origin of the Leda clay, the 

 upper part of the Erie and other stratified clays, the Saxicava 

 and other sands and beaches. However, accordins; to the lilacial 

 theory, much of the stratification of the deposits took place in 

 lakes and rivers dammed up by the glacier itself, without so 

 ijrreat a subsidence of the continent as the extreme icebero- theo- 

 rists would have. 



Distribution of the Northern Drift. — Let us now examine 

 what evidence, aiding the elucidation of the history of the Great 

 Ice Age, can be derived from the study of the region of Lake 

 Ontario. In doing this, however, it will be necessary to go some- 

 what out of the locality of our immediate study. 



The so-called ice-cap of the northern hemisphere was con- 

 fined principally to the region of the North Atlantic Ocean. In 

 America, Professor Whitney states, as the result of extended 

 observation, that there is no evidence of an ice age at low levels 

 along the Pacific Coast, except along the sea, at such elevations 

 as could be glaciated by floating ice during a slight subsidence 

 along the coast of Vancouver's island, on an adjacent coast of 

 the mainland. The southern limit of the northern drift on the 

 eastern side of tlie Rocky Mountains may be approximately 

 designated by a line drawn from the head waters of the Saskat- 

 chewan river to the mouth of the Missouri river, thence to the 

 centre of Ohio, through Pennsylvania and Nevv York, to northern 

 New Jersey. 



