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 example, 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 highlands as it was in the Quaternary age.
In Chapter VI., which forms a résumé 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 Archæan, 32,000 feet of Palæozoic, 30,000 feet of Mesozoic, 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 performed 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 upheaval 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 valuable 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 generalization which he makes in this part of the book, we are, however, compelled 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
