MOUNT HOLMES BYSMALITH. 



65 



grained to aphanitic texture. It is not markedly porphyritic. The variations 

 in texture occur near the margin of the body, where they bear a definite 

 relation to the contact plane. They are accompanied by a slight change in 

 the chemical composition. The uniformity in the character of the mass 

 indicates that the whole body was one magma, erupted at one time. Its 

 mineral composition is seen with the microscope to be quartz and alkali 

 feldspar with biotite, corresponding to that of biotite-granite. Its chemical 

 composition is shown by the following analyses, one of which represents 

 the main mass of the rock; the other, which is more siliceous, is of rock 

 from near the margin of the bysmalith, at Echo Peak. 



Analyses of dacite-porphyry and rhyolite-felsite. 



[Analyst, J. E. Whit6eld.] 



Constituent. 



Si0 2 



Ti0 2 



A1 2 3 



F. 2 3 



FeO 



MnO 



MgO 



CaO 



Li0. 2 



Na»0 



K 2 



P 2 3 



S0 3 



H 2 



Total 



(77) 

 Mount 

 Holmes. 



69.54 



None. 



17.95 



2.50 



.22 



None. 



.50 



1.80 



Trace. 



4.30 



1.21 



None. 



.37 



1.96 



100. 35 



(87) 

 Echo Peak. 



74. 51 



None. 



14.83 



1.09 



Trace. 



Trace. 



.47 



.81 



.02 



4.38 



2. 72 



Trace. 



.24 



.92 



99.99 



The chemical composition corresponds to that of a granite high in soda. 

 The main mass has a chemical composition just on the border line between 

 soda-rhyolite and dacite, while the marginal portion has the composition of 

 soda-rhyolite. As already noted, its texture is not markedly porphyritic, so 

 that it stands between a distinct porphyry and a felsite. It is, consecpiently, 

 difficult to give it a name that will not be open to criticism. We have called 

 the main mass dacite-porphyry. 



In thin section the rock appears as a gray, very fine-grained mass with 

 abundant minute specks of biotite. Under the microscope it is seen that 



MON XXXII, PT II 5 



