8 J. Bar veil — Relations of Subjacent Igneous 



the reasons for regarding these stocks as the seat of a . 

 Pliocene batholith which here may have broken through 

 its roof. Thus there are strong inferences that regional 

 batholithic invasion has occurred under a vast region in 

 Idaho and western Montana, a larger portion being con- 

 cealed than has become revealed by erosion. 



Another line of evidence of broader character is found 

 in the lava flows and volcanic breccias which spread over 

 the greater part of the Cordillera. These imply the 

 presence of widespread magma with depth as the sources 

 of this surface material. But the study of regions of 

 exposed batholiths shows that gravitative differentiation 

 proceeds, granites coming to occupy the upper parts of 

 the magma chamber and invading even the older and 

 more basic surface materials. This is the history of the 

 Laurentian cutting the Keewatin flows, and of many 

 younger examples down to the Boulder batholith, whose 

 quartz monzonite comes to metamorphose and cut the 

 somewhat older andesites and basalts. The evidence of 

 exposed regions suggests, therefore, that Tertiary granite 

 should occur widely in depth under much of the Cordil- 

 leran province. 



Still another line of inference, and the one of broadest 

 character, is that founded on the geodetic evidence. 

 Cordilleran seas occupied, through much of Paleozoic and 

 Mesozoic times, portions of what is now one of the great 

 plateau regions of the world. ^Mountain ranges were 

 raised, and crustal warpings took place from time to 

 time then as now, but the general average of the Cordil- 

 leran province remained near sea-level, as shown by the 

 preservation, through these earlier eras, of sediments 

 mostly of marine origin. The principle of isostasy re- 

 quires that the subcrustal regional densities must have 

 corresponded with this attitude, giving conditions of 

 equilibrium. 



Now, however, the geodetic observations show that a 

 condition of regional isostasy prevails, comparable to 

 that in other portions of the United States, notwith- 

 standing the mountainous as well as plateau character 

 of the province. Furthermore, there is evidence that 

 in the Great Basin the depth of is o static compensation 

 is probably shallower than in any other part of the 

 United States. This contrast of the present and the 

 past is strongly suggestive of a thinning of the crust 

 and a decrease in subcrustal density in the Cordilleran 



