232 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



magma was not homogeneous at the time of its solidification. To that fact is 

 doubtless to be related the confused, rapid crystallization of the essential 

 mineral constituents. 



ABNORMAL HORNBLENDE-BIOTITE GRANITE. 



The biotite granite of sill C graduates downward into a rock of similar 

 habit, with hornblende added to the list of essential constituents (Phase 4). 

 The amphibole resembles that of the unaltered gabbro of the Purcell sills. 

 The structure of this hornblende-biotite granite changes rapidly and appar- 

 ently irregularly from the micrographic to the hypidiomorphic-granular. The 

 top zone of sill D is composed of the same rock type. No chemical analysis 

 has been made of it. The specific gravity of a specimen from sill is 2-765,. 

 being slightly greater than the average for the overlying biotite granite. 



INTERMEDIATE ROCK TYPE. 



Underlying the hornblende-biotite granite in both sill C and sill D, and 

 underlying the biotite granite in sill A, are zones composed of a rock which 

 combines features of granite and gabbro (Phase 5). It is, in fact, a rock 

 directly transitional into the dominant gabbro of the Purcell sills. A specimen 

 illustrating this intermediate rock was collected at a point 200 feet below the 

 upper contact of sill C, and has been analyzed. 



Macroscopically this phase is much like the usual gabbro of the sills. It 

 is a dark, greenish-gray, granular rock of basic habit. Its essential minerals are 

 hornblende, biotite, and andesine; the accessories, quartz, orthoclase, titanite, 

 titaniferous magnetite and apatite. The secondary minerals are zoisite, kaolin 

 and epidote. The structure of the rock is in general the hypidiomorphic-granular, 

 but local areas of micropegmatite are common in the section. 



The total analysis of this phase (specimen No. 1140) by Prof. Dittrich gave 

 the following result: — 



Analysis of Intermediate Roch (Phase 5) of Moyie Sills. 



Mol. 



Si0 2 52-63 -877 



TiO- -62 -008 



ALA 16-76 -165 



Fe,0 3 2-86 -018 



FeO 10-74 -149 



MnO.. -38 -006 



MgO 4-33 -108 



CaO 6-17 -110 



Na a O Ml -023 



K,0 2-29 -024 



H,0 at 110°C -12 



H 2 above 110°C 1-17 



P,0 K -33 -002 



C0 2 -10 



99-91 

 Sp.gr (corrected value). 2-954 



