CHAP. xn. GEOLOGICAL PRINCIPLES. 127 



logical Survey and by many local geologists, and is uni- 

 versally accepted by all who have studied the evidence. 

 The great outlines of the phenomena of the ice age in 

 our islands are now as thoroughly well established as any 

 of the admitted conclusions of geological science. In 

 our own country the ice extended more or less com- 

 pletely over the whole of the midland counties and as 

 far south as the Thames Valley. 



When we cross the Atlantic the phenomena are 

 equally remarkable. The whole of the northeastern 

 United States and Canada were also buried in an ice- 

 sheet of enormous thickness and extent. It came south- 

 ward as far as New York, and inland, in an irregular 

 line, by Cincinnati, to St. Louis on the Mississippi. The 

 whole of the region to the north of this line is covered 

 with a deposit of drift, often of enormous thickness, 

 while embedded in the drift, or scattered over its surface, 

 are numbers of blocks and rock-masses, often formed of 

 materials quite foreign to the bed-rock of the district. 

 These erratics have in many cases been traced to their 

 sources, sometimes 600 miles away, and the study of 

 these, and of the numerous grooved and striated rocks, 

 show that the centre of dispersal was far north of the 

 Alleghanies and its outliers, and, as in the case of Ire- 

 land, must have consisted of a huge dome of ice situated 

 over the plateau to the north of the Great Lakes, in what 

 must have been an area of great snow-fall combined with 

 a very low temperature. The maximum thickness of 

 this great ice-sheet must have been at least a mile over a 

 considerable portion of its area, as glacial deposits have 

 been found on the summit of Mount Washington at an 

 altitude of nearly 6000 feet, and the centre of motion 



