Reviews — Dr. O. F. Wright — Ice Age in North America. 567 



Croll's views on the cause of the Glacial Pei'iod and of astronomical 

 calculations respecting Glacial chronology. On the other hand, the 

 views on the former extent and action of land-ice, enunciated by 

 Agassiz in 1840, have gained almost universal adherence, and there 

 remain very few geologists whose opinions can be said to be seventy 

 years or more behind the times. 



In the preliminary account of existing glaciers and ice-sheets a good 

 deal of new material has naturally been gathered from researches in 

 Greenland, Central Asia, and the Antarctic regions. In Alaska, again, 

 interesting instances have been recorded of the advance and retreat of 

 the glaciers bordering Yakutat and Glacier Bays. In the former bay 

 a remarkable advance in the ice has been attributed by Professor Tarr 

 to the effect of an earthquake in 1899. By that disturbance a portion 

 of the coast was elevated 47 feet, and thereby large quantities of snow 

 were probably shaken from the more elevated peaks upon the head of 

 the glaciers. Interesting, too, are the accounts of the Muir Glacier, 

 the ice from which nearly filled Glacier Bay a. century ago. Owing 

 to the present recession a buried forest with many upright stumps of 

 trees has been partially exposed beneath sand and gravel, and the 

 author remarks, " There can be no doubt that, after the accumulation 

 of sand burying the forest, the glacier advanced for a great distance 

 over it, attaining a thickness at that point of two or three thousand 

 feet." This leads to some remarks on the capacity of ice to move over 

 such deposits without disturbing them. No doubt a great deal depends 

 on the form and composition of the land-surface over which the ice 

 passed, whether smooth or irregular, evidences of drag and disturbance 

 being more often seen where the surface was uneven or where boulder- 

 clay rests on mixed strata of clay and gravel. 



Greenland, regarded as an island of continental proportions and the 

 most important accessible field for glacial observations, is almost 

 wholly covered by an ice-sheet, having an area of about 575,000 square 

 miles, a length of 1,500 miles, and a width of 700 miles. The author, 

 however, remarks that "The absence of glacial phenomena north of 

 the range of mountains which forms the southern boundary of Alaska, 

 and over the adjacent plains of Northern Siberia, completely disproves 

 the once current theory that the glacial period was characterized by 

 a vast ice-cap extending in all directions from the pole ". 



The Ice Age in North America, as in Europe, "began and ended 

 in a great number of local glaciers which became confluent and 

 continuous only during the middle of the period." The area covered 

 by the ice at its maximum extension is shown in fig. 62, and other 

 maps indicate in more detail the southern limit of the ice-sheet 

 and drift. 



On the coast of Maine the ice is reckoned to have been more than 

 1,500 feet thick, while over the central portions of the glaciated area 

 in North America it may have been as much as three miles. This 

 latter calculation is based on the evidence of movement of ice 

 from Labrador to the southern part of Illinois, a distance of about 

 1,600 miles. It is estimated that there must have been an average 

 slope of 10 feet to the mile, and that the movement must have been 

 produced mainly by the simple accumulation of ice. 



