60 H. F. Picid — Studies of Muir Glacier. 



Eruptive Rockh. 



Slides of all the eruptive rocks found in the district, ground 

 by the United States Geological Survey, through the courtesy of 

 the director, were sent to Dr George H. Williams for examina- 

 tion. It is much to be regretted that, owing to a misunderstand- 

 ing, no field-notes were sent him. His paper accompanies this, 

 and to it the reader is referred for the nature of the rock species 

 under discussion (supplement ii). There are two main classes 

 of igneous rocks in the district : ( 1 ) ancient eruptives occurring 

 in large masses, classed as diorites by Dr Williams, and (2) more 

 recent eruptives, occupying dikes generally of small size, which 

 cut both the se'dimentary and the older eruptive rocks. 



The Diorites. — The northern and northeastern tributaries of 

 Muir glacier have brought down on their moraines great quanti- 

 ties of diorite, derived from the mountains adjoining the upper 

 j)ortions of their valleys. These diorite mountains lie just north 

 of those of argillite which form the southeastern boundary of the 

 glacial amphitheater. • I saw the rock in place only from a dis- 

 tance, and had no opportunity to examine the contact between 

 it and the argillite. This renders it impossible to state which of 

 the two is the older. No evidence of- shore conditions is observ- 

 able in the vicinity of the contact, nor is there any evidence per- 

 ceptible to the eye that the argillites derived any of their material 

 from the diorite, affording a slight negative evidence in favor of 

 the diorite being the younger. To the eye the diorite seems to 

 consist of nearly black hornblende and a white plagioclase, the 

 hornblende largely predominating. A distinctly foliated arrange- 

 ment is often observable, simulating a rough gneissic structure. 

 In the main it is fine grained, but sometimes quite coarsely 

 crystalline. 



Quartz- diorite. — Another great plutonic mass, having a very 

 different appearance from the last and completely separated from 

 it in the region examined, is described as quartz-cliorite by Dr 

 Williams. Its outcro}) is fan-sliaped, running out toward the 

 east, but having a great development toward the northAvest and 

 west. Nunataks G and H are composed of it, and also the moun- 

 tains toward the west and northwest as far as the eye can reach 

 the peculiar light tint of the rock making it easily recognizable 

 at some distance. The moraines that come down from that 

 direction are almost entirely composed of this material. Totlie 



