part 1] THROUGH THE ANDES OE PERU AND BOLIVIA. 17 



In discussing the general history of the complex, the three phases 

 of intrusion, which may conveniently be termed the granodioritic, 

 the dioritic, and the granitic, must be studied independently, 

 although it will be seen that they are, in all probability, intimately 

 connected ; the magmas from which they were derived, moreover, 

 may at one time have been coexistent in the same deep-seated 

 reservoir. 



In order that many of the observed phenomena may be more 

 readily explained, it is necessary to assume that a certain degree of 

 magmatic differentiation had been arrived at prior to the first in- 

 trusion. The granodiorite may then be regarded as representing 

 more or less the normal composition of the original magma, from 

 which the granite and diorite were derived as differentiation- 

 products ; the former richer in silica and alkalis, and the latter 

 richer in ferromagnesian elements and lime. 



In describing the granodiorite as the first phase of intrusion, it 

 is important to note that this rock contains numerous basic patches, 

 which in some places become so abundant that they completely 

 alter its appearance. These are regarded as portions of dioritic 

 magma, already in part consolidated, which have been incorporated 

 in the residual magma and have undergone complete recrystal- 

 lization, either in the process of digestion, or by the subsequent 

 metamorphism to which the entire series has been subjected. 

 The deep-seated origin of these xenoliths is assumed from the 

 fact that no outcrop of basic rock earlier than the granodiorite was 

 anywhere discovered. 



The normal granodiorite, with its xenoliths of diorite, appears, 

 next, to have been invaded locally by a highly-acid magma represent- 

 ing the other extreme end of the differentiation series. The 

 result has been an admixture of the two types with the pro- 

 duction of a hybrid rock of peculiar character. For the sake of 

 distinction, I have termed this the abnormal granodiorite. 



Beyond the limit to which actual assimilation of the earlier rock 

 took place, there appears to be a considerable zone of contact- 

 metamorphism, but the effects due to the one cause or the other 

 are not readily distinguishable ; they are, naturally, intimately 

 connected, and therefore differ only in degree. Where the basic 

 xenoliths of the original rock, however, are seen to retain their 

 individuality, it may be assumed that the limit has been passed. 



(At one time I thought that the acidification of the granodiorite 

 might be due to assimilation of an original quartzite cover, but the 

 appearance of the quartz-mosaic under the microscope makes it 

 quite improbable that such was the case.). 



Before discussing the origin of the xenoliths, some mention must 

 be made of the metamorphism which they have undergone since their 

 incorporation in the granodiorite. Three factors have to be taken 

 into consideration. First, the effects produced by their initial capture 

 and pai-tial digestion ; secondly, the effects due to the approach of 

 the acid magma ; and last, the effects of dynamic metamorphism. 

 It is by no means easy to discriminate between them. 



Q. J. G. S. No. 301. c 



