730 REVIEWS 



general discussion of the theory of isostasy, with illustrations drawn 

 from other regions of elevation or subsidence. 



Before all, the physiographic features of the Great Valley are fully 

 considered and the several parts in some detail. The main geological 

 features are described and the evidence of several deep wells is brought 

 in to show the composition of the strata underlying the region. 



The geological history of the region is briefly and concisely 

 related. This leads directly to the main theme — the principles of 

 isostasy, for the Great Valley has an area in which there has been pro- 

 found and progressive subsidence accompanied by great deposition. 

 The literature relating to the theory is compactly brought together 

 and summarized. The theory is then examined in the light of the 

 facts derived from a study of the particular area. 



The Great Valley represents a limited area in which 2000 feet of 

 sediments were deposited at the same time that the rim was being 

 raised. In explanation of the attendant phenomena the hypothesis of 

 isostasy is not only regarded as unnecessary, but the facts presented 

 are thought to directly oppose the idea of isostatic subsidence. A 

 consideration of the essential features of isostasy shows it to be merely 

 an hypothesis of readjustment and not of initial movement. It can 

 operate only after conditions have become unstable. So far as applied 

 to the Great Valley it is shown conclusively that the orogenic move- 

 ments elevating the Coast Range began before the "formation of the 

 valley and are consequently independent of any loading and subsi- 

 dence." A little latter the Sierra Nevada range began to rise and in 

 the great syncline between were deposited shallow water sediments to 

 a depth of 2000 feet. Other facts are enumerated that are believed to 

 be opposed to the hypothesis of isostatic movement in the valley. 

 Several Californian areas are also brought in as presenting phenomena 

 perfectly inexplicable upon the isostatic principle. After passing them 

 the general discussion of the theory as a whole is reached. 



In discussing the data upon which the theory has been made to 

 rest specific cases are taken up. McGee's consideration of the Gulf of 

 Mexico and the great Mississippi embayment receive particular atten- 

 tion. It is not, however, with the acumen shown in the other parts of 

 the memoir. Some of the very objections urged against the region 

 having subsided by loading are themselves so manifestly defective that 

 they have the very opposite effect from that intended and naturally 

 strengthen the hypothesis rather than weaken it. One point in partic- 



