316 THE POPULAR SCIENCE MONTHLY, 



climate is very cold, and yet the precipitation is sufficient, with the 

 average temperature, to form continental glaciers of equal dimensions 

 with any indicated by the records of the past, and, so far as we can 

 judge, no other condition is necessary for the extension of the glaciers 

 of Greenland southward to Labrador, the Canadian highlands, and the 

 hills of New England, than a depression of temperature sufficient to 

 congeal and retain the moisture which now flows away nearly as fast 

 as it falls. With the arrest of the flow of the St. Lawrence, for exam- 

 ple, and its accumulation year after year as ice and snow, it would not 

 require many centuries to pile the ice as high on the Canadian high- 

 lands as it was in the Quaternary age. 



In Chapter VI., which forms a resume of the stratigraphical geology, 

 Mr. King refers in his graphic and felicitous way to the conditions of 

 deposition of the 120,000 feet of sedimentary accumulations which 

 form the different groups we have reviewed. The tabular presentation 

 of the stratigraphy (page 544), giving at one view the relations of the 

 50,000 feet of Archaean, 32,000 feet of Palaeozoic, 30,000 feet of Meso- 

 zoic, and 15,000 feet of Cenozoic rocks, is the most comprehensive 

 and impressive section which has ever been published, and one that 

 shows at a glance the magnitude of the task which Mr. King has per- 

 formed in the correlation and coordination of such a vast amount of 

 material. 



In the last two chapters of his volume Mr. King discusses at great 

 length the genesis and relations of the Tertiary volcanic rocks, and 

 more briefly the classification of the mountain-ranges and lines of up- 

 heaval which traverse his field of exploration. These chapters, though 

 of great interest to the geologist, will perhaps not attract the general 

 reader. We would, however, specially commend them to students of 

 lithology and physical geology, as they contain a vast amount of val- 

 uable information on what have been made subjects of special study 

 by Mr. King. There was no part of his duty for which he was better 

 prepared than that he has done here ; and perhaps none in which he 

 has acquitted himself more creditably. The most striking generaliza- 

 tion which he makes in this part of the book, we are, however, com- 

 pelled to question. This is a new theory of the origin of vulcanism. 

 Most geologists of the present day believe that the crust of the earth 

 is thicker than was once supposed, and that its thickness is increased 

 by the effect of pressure which holds in coerced rigidity a zone of 

 greater or less depth, which is heated above the point at which it 

 would fuse and flow under the pressure of the atmosphere only, and 

 that local relief of this pressure would permit a greater or less mass 

 of highly heated matter below to burst into fluidity, and perhaps find 

 its way to the surface. Mr. King proposes erosion as a sufficient cause 

 for the relief of pressure and the production of volcanic phenomena ; 

 but some facts suggest themselves which seem to be incompatible with 

 this theory, viz. : 1. Erosion is so slow on an average 3,000 years 



