548 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



The voluminous volcanic rocks that accumulated prior to the batho- 

 lithic intrusions with their abundant andesites and keratophyres are clearly 

 of the eugeosynclinal andesite province, according to the writer's classifica- 

 tion. 



All the post-batholithic volcanic rocks that occur within the orogenic belt 

 of the Andes are of the andesite orogenic belt province and are very similar 

 to the eugeosynclinal rocks except that they lack the spilites and kerato- 

 phyres. The rows of great stratovolcanoes which are very late in the 

 general Cenozoic volcanic sequence are conspicuous for their alignment 

 and dominant central vent character, but in terms of composition are 

 must like the orogenic belt andesites with which they are closely asso- 

 ciated. The melting of downward extended roots of mountains in the 



orogenic belts has been visualized as the source of the large volumes 

 of andesite, but since large volumes of basalt, both olivine and tholeiitic, 

 are also erupted in the orogenic belt with the andesite, we must provide 

 for the rise into the granitic crust of such magmas from the subcrust. The 

 basalt is generally more prominent in the late volcanic stages than as 

 alternating extrusions with the andesite, and this fact should be kept in 

 mind. Also it should be noted that by theory the roots of orogenic belts 

 are thought to melt to form granodiorite and granite in great volume for 

 the batholithic cycle, and on the other hand, some petrologists have 

 postulated that roots melt in volume to provide the magma for the ande- 

 sitic extrusions. Since the composition of granodiorite is considerably 

 different, the same conditions, exactly, cannot exist for both. 



