Pirsson — Petrography of Tripyramid Mountain. 413 



tion the mineral varies in some crystals from Ab 2 An 3 to Ab 6 An 3 , 

 that is from calcic andesine to rather basic labradorite. 

 The average of these two is Ab T An a , while the average feld- 

 spar calculated from the analysis in obtaining the norm given 

 below is Ab 8 An 6 , and this close approximation tends to show 

 that the average feldspar must have about this composition. 

 Only occasionally is it zonally built. 



The orthoelase and quartz are present in small amounts not 

 exceeding two or three per cent each. They are not inter- 

 grown, as is sometimes the case in rocks of this group, but 

 occur independently, filling minute angular interspaces between 

 the other minerals. In the calculated norm given below there 

 is 10 per cent of orthoelase and 2 per cent of olivine and no 

 quartz. This is because the biotite has been split up into oli- 

 vine and kaliophilite, and the latter has used up the free silica 

 of the quartz and become orthoelase. If we reverse this pro- 

 cess and consider the biotite to have essentially the simple 

 formula K.,(Mg,Fe) s Al 1 Si 1 0„, this will split up into 2KAlSi0 4 + 

 (Mg,Fe) 2 Si0 4 , that is into kaliophilite and olivine. With these 

 data it may readily be calculated that the 10 per cent of ortho- 

 elase and 2 per cent of olivine of the norm are equivalent to 

 6*11 per cent biotite, 3*12 per cent quartz, and 2 , 78 per cent 

 orthoelase = 12 - 02, which is what the rock modally contains. 



Mode. — By observation of the section and use of the data 

 given above the actual mineral composition of the rock is as 

 follows : 



Iron ore 11*0 



Apatite 4-4 



Biotite 6-0 



Augite 5 "6 



Hypersthene 10*0 



Plagioelase 56*5 



Orthoelase .. 3-0 



Quartz 3-0 



Rest 0-5 



Thus the percentage of feld- 

 spathic components, as compared 

 with the ferromagnesian ones, is 

 about two to one and thus clearly 

 places the rock in the feldspathic 

 members of the gabbro family. 



Total 100-0 



Texture. — Although in the hand specimen the rock appears 

 quite even-granular, in the section it is seen to be inequigranu- 

 lar and seriate, and eonsertal in fabric. That is, while many Of 

 the grains are approximately equal in size, they grade down 

 into much smaller ones, and all are closely fitted together, or con- 

 serted. The most characteristic feature of the fabric is that 

 produced by the rather long and large sections of the tabular 

 feldspars diversely spread through the rock, with the other 

 components and the smaller feldspars filling in the spaces 

 between. 



