420 Pirsson — Petrography of Tripyramid Mountain. 



It is, therefore, dosalic, perfelic, docalcic, and presodic, and 

 its coordinates are 2.5.4.3. The amount of biotite is negli- 

 gible and hence the mode is a normative one. The texture 

 being granular it is grano-hessose. In prevailing qualitative 

 systems it is an olivine-gabbro of feldspathic type, or rather, 

 one should say, it would be pronounced such if its association 

 with the monzonite and syenite were unknown, or unsuspected. 

 By those petrographers who hold rigidly to genetic lines in 

 classification, if these facts concerning it became known, it 

 would, supposedly, be classed as an essexite. 



Quartz-Syenite Aplite (Liparase). 



It has been previously mentioned that where the rock sur- 

 faces of Tripyramid Mountain are well exposed they are found 

 to be cut by narrow dikes, or dikelets, of aplitic character. 

 While there is some variability in the relative proportions of the 

 minerals composing these rocks, and in their texture, it is not 

 so great but that they can all be placed in one well-defined 

 group — they are quarts-syenite aplites. Also, they are rocks 

 of such very simple composition that only a very brief descrip- 

 tion of them is necessary. 



Megascopic.— Phanerocrystalline ; very fine-grained, but not 

 dense ; usually flesh-colored but varying from whitish to pale 

 reddish brown ; persalic and predominantly feldspathic ; 

 faintly dotted at times with minute, blackish specks ; appar- 

 ently equigranular in texture and of firm habit ; fracture not 

 easy and hackly ; often rather weathered* 



Microscopic. — Alkalic feldspar essential, sometimes also 

 quartz ; iron ore, biotite and zircon accessory, sometimes also 

 quartz. 



The alkalic feldspar is mostly orthoclase, with some micro- 

 perthite intergrowth of albite ; soda-lime feldspar was not 

 observed. The feldspar appears in the slide in rather equi- 

 dimensional sections, which tend to be square in outline, 

 indicating a roughly cuboid habit. The interspaces are filled 

 with quartz which is completely anhedral in consequence ; in 

 those cases where the quartz is more abundant it tends to a 

 rudely micrographic fabric, in that the various interspaces 

 between several feldspars are filled by a quartz mass of uniform 

 orientation. An occasional grain of iron ore, sparsely sprinkled 

 flakes of light brown biotite, generally altered to chlorite, and 

 a rare crystal of zircon complete the list of minerals. So far 

 as the feldspars, which are the predominant components, are 

 concerned the fabric tends to be equigranular, and equant. If 

 the quartz be also considered the fabric is seriate, consertal, 

 tending in some cases to graphic. 



