REPORT OF THE CHIEF ASTRONOMER 359 



SESSIONAL PAPER No. 25a 



are constant and important differences in composition between the stocks and 

 the batholith, so that their sharp distinction is necessary in a detailed survey 

 of the region. 



Dominant Phase. — Macroscopically, the dominant phase of the Coryell 

 batholith is a medium-to coarse-grained, occasionally somewhat porphyritic, 

 light reddish to brownish-pink rock of typical syenitic habit. It is usually fresh 

 and is generous to the collector of fine specimens. Greenish-black, lustrous 

 hornblende, and brilliant biotite are the visible femic constituents; they occur 

 scattered through the feldspars which are both striated and unstriated. In 

 thin section the principal feldspar is seen to be microperthite, associated with 

 much sodiferous orthoclase and subordinate plagioclose, averaging andesine, 

 Ab. An 3 . A few small idiomorphic crystals of diopsidic augite, a little inter- 

 stitial quartz, rare grains of allanite, and the usual apatite, titanite, and 

 (probably titaniferous) magnetite are accessory. 



The structure is eugranitic; the order of crystallization is apparently: — 

 Apatite, magnetite, titanite, augite, plagioclase, hornblende, biotite, alkaline 

 feldspars, and quartz; with some overlapping in the generation-periods of the 

 hornblende and plagioclase. The specific gravity of six fresh specimens ranges 

 from 2-648 to 2-729, with an average of 2-675. 



This chief phase of the batholith has not been specially analyzed for the 

 present report, but the following analysis of a type specimen (collected at a 

 point north of Record mountain) has been made by Professor Dittrich for 

 Mr. Brock: — . 



Analysis of Coryell syenite. 



Si0 2 62-59 



Ti0 2 0-54 



AlA 17-23 



Fe 2 3 1-51 



FeO 202 



MnO tr. 



MgO 1-30 



CaO 1-99 



Na 2 .. 5-50 



K 2 6-74 



P 2 6 Oil 



H,0 (direct) -30 



C0 2 trace 



CI trace 



S0 3 trace 



99-83 



Another typical specimen collected by the present writer on the 6,820-foot 

 summit about four miles north of the Boundary and thus well toward the 

 center of the batholith has been studied quantitatively according to the Bosiwal 

 method. The weight percentages of the constituents were found to be approxi- 

 mately a 9 follows: — 



