254 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



(1425), with more hornblende intergrowths. The pyroxenes have the char- 

 acteristics of those in the gabbro. The microstructure is coarser than that 

 of the 20-foot dike, which is represented by specimens 1407, 1405, the 

 finer-grained form (1405) being about grade 28. Rock 1400 has less ferro- 

 magnesian minerals and resembles in outward appearance the fine-grained 

 rock from the top of the core (1414). Its structure is shown in PI. XXXIV, 

 fig. 2. 



Two small dikes of similar quartzose diorite-porphyry cut the aphamtic 

 zone at the base of the west spur. They are dense and crystalline, without 

 noticeable phenocrysts (1385, 1386). The coarser of the two resembles 

 the rock last mentioned in microstructure, and is about grade 27. It has 

 the same mineral composition as the finer-grained rock which occurs in a 

 dike 1 foot wide. The latter (1385) differs from it slightly in microstruc- 

 ture, and is about grade 18. Its phase of crystallization is specially inter- 

 esting. In thin section it consists of lath-shaped feldspars of various sizes, 

 and small grains of feldspar, with considerable quartz, which has a micro- 

 poikilitic structure. The groundmass also contains magnetite, biotite, 

 augite, and altered hypersthene. Its most notable characteristic is the 

 development of small phenocrysts of orthoclase in Carlsbad twins with 

 irregular outlines. The outer portion of these crystals incloses the ferro- 

 magnesian minerals, and occasionally plagioclase. There are few pheno- 

 crysts of plagioclase. The orthoclase appears to belong- to the period of 

 crystallization when the quartz formed in poikilitic individuals of nearly the 

 same size as the orthoclase, but inclosing more plagioclase. The outline 

 between the quartz and the orthoclase is irregular. Thus the porphjnitical 

 orthoclase crystals are younger than the lath-shaped plagioclase and ferro- 

 magnesian minerals of the groundmass of the rock. The rocks are banatite- 

 porphyry. 



There are still more quartzose facies of the magma, which cut the 

 dioritic facies of the middle spur in dikes or veins. A coarsely crystalline 

 variet} T (1428) forms a 4-foot dike cutting the main body of rock (1429), 

 which is somewhat finer grained and has been described. The dike rock 

 is about grade 40, and is like the main part of the spur lower down (1425), 

 being of the same degree of ciystallization. The microstructure is like 

 that of the latter rock, but there is more quartz and feldspar and less 



