DIFFERENTIATED INTRUSIVE SHEET. 



83 



Analyses of different parts of the sheet southeast of Electric Peak. 



Constituent. 



1. 



2. 



3. 





50. 59 



.80 



11.53 



1.83 



7.64 



.06 



.17 



11.27 



8.79 



.10 



.03 



2.27 



2.33 



Trace. 



.48 



Trace. 



None. 



.21 



1.76 



52.10 



.79 



16.34 



3.84 

 6.82 



51. 85 



.79 



15. 54 



3.50 



6.95 



.01 



.03 



5.49 



5.40 



.01 



Trace. 



3.73 



3.89 



.10 



.64 



.20 



.18 



1.78 



TiO, . 



Fe-0 , 



FeO 



NiO . 



MnO -. 



Trace. 

 4.33 

 4.73 



fMg"0 



CaO 



BaO 



SrO 



K,0 





4.02 

 4-20 



• 13 



■ 68 



• 24 



• 22 



Li.,0 



p.,0 , 



CI 



H 2 below 110° 



H^O above 110° 



Total 



99. 86 



100. 18 

 .05 



100. 09 

 .04 



Less for CI 







100. 13 



100. 05 



The chemical composition of the sheet is not like that of any of the 

 igneous rocks of Electric Peak and Sepulchre Mountain, being lower in 

 silica and proportionately higher in alkalies. It approaches the composition 

 of shoshonite (Chapter IX), and indicates that the magma is a more highly 

 differentiated form of the general magma. The two parts of the sheet cor- 

 respond in chemical composition more closely to absarokite and shoshonite 

 (q. v.), and are the results of a differentiation in which the separation was 

 according to mineral molecules, as it was actually the settling of pyroxene 

 crystals. The ratio of the alkalies to one another in the two parts is nearly 

 constant, indicating that they remained in combination. 



The upper part of the rock, 10 feet from the tojo, consists of feldspar, 

 most of which is plagioclase, and is striated. It is partly in tabular or 

 equidimensional crystals, with allotriomorphic outlines, partly in lath-shaped 

 and rectangular crystals, more nearly idiomorphic and generally of smaller 

 size. The extinction angles are low, often nearly zero, indicating oligoelase. 



