GEOLOGY OF THE HAYSTACK STOCK 223 



peripheral acid varieties, while D, E, and F are -the basic central 

 varieties. This table shows that quartz is present in the outer border 

 of the stock in largest amount, and decreases toward the center of 

 the stock. Orthoclase is present in every case, but is most abundant 

 in the facies not far from the periphery. Plagioclase is present in 

 approximately the same amount in all varieties, but is more calcic in 

 the central ones. Pyroxene is least abundant near the periphery, 

 and most abundant in the most basic facies near the center of the 

 stock. Hornblende, in part probably secondary to pyroxene, is 

 relatively abundant only in the most acid portion near the periphery. 

 Olivine is abundant only in the most basic varieties near the center. 

 Biotite and magnetite are fairly uniform in quantity. 



The order of crystallization of the minerals of the stock, briefly 

 summarized, is as follows: 



1. Apatite, crystallized first, since it occurs in all the other minerals. 



2. The ferromagnesian minerals, pyroxene, hornblende, biotite, 

 magnetite, and olivine, crystallized early but mainly after apatite 

 had formed. 



3. Plagioclase, crystallized after apatite, after and along with 

 the ferromagnesian minerals. 



4. Orthoclase and quartz, crystallized together after plagioclase 

 and the ferromagnesian minerals had formed. 



GRANITE (oMEOSE) VEINS CUTTING THE HAYSTACK STOCK 



Granite veins one-haK inch to six inches wide cut various rocks 

 of the Haystack stock. These occur at the base of the cliff of the 

 southeast of Blue Lake, on the southwestern slope of Baboon Moun- 

 tain near the summit, on the wagon road from Cowles to the Yellow- 

 stone National Park about two hundred and fifty yards east of the 

 divide in Haystack Basin, and at many other localities. The granite is 

 everywhere from medium to coarse-grained, and is lighter colored than 

 the rock it cuts. The granite is composed of feldspar and quartz 

 with a little biotite and hornblende. The veins do not appear to be 

 persistent in length, since they cannot be traced continuously for 

 any great distance. A specimen from a 2-inch vein cutting a very 

 dark fine-grained quartz-bearing diorite (C) on the southwest slope 

 of Baboon Mountain is a pinkish gray, medium-grained rock. 



