STOCK ROCKS AND APOPHYSES. 97 



some titanium. Apatite is more abundant in the more micaceous rocks. 

 The same is true of zircon 



Secondary pyroxene occurs in porphyries within the metamorphosed 

 sandstones, apparently derived from hornblende. The feldspar and biotite 

 phenocrysts are quite fresh, and the general microstructure of the ground- 

 mass is normal for a hornblende-mica-andesite-porphyry. The augite 

 substance is colorless and compact, with pyroxenic cleavage. It occupies 

 sjDaces with hornblende outlines in cross section, the plane of symmetry for 

 both minerals being the same No pyroxene outlines were observed. Its 

 refraction and double refraction are high, and the extinction angle is large. 

 It sometimes forms irregular grains and minute aggregations not immediately 

 connected with a hornblende crystal. In some cases the hornblende is not 

 entirely changed to pyroxene. The process by which this alteration took 

 place has not been made out. 



II. THE STOCK ROCKS AND APOPHYSES. 



The diorite forming the body of the stock varies in the size of its 

 crystals. Most of it is medium grained, consisting of clusters of feldspars, 

 and others of ferromagnesian minerals, from 5 mm. to 2 mm. in diameter, 

 and smaller. The coarsest is shown in fig. 1 of PI. XVII, photographed 

 natural size. The size of the component crystals is smaller than that of the 

 clusters, and is from 1 mm. to 2 mm. A medium-grained form is shown in 

 fig 2 of PL XVII. The grain sinks to fine grained and to microcrystalline. 

 The variation in grain is gradual in some parts of the mass and rapid in 

 others. The finer-grained portions are darker colored. While a gradual 

 transition from coarse to fine grained and from light to dark-colored rock 

 can be traced in some places, the two extremes are in juxtaposition in 

 others, with a sharp line of demarcation between, veins of the light-colored 

 rock cutting the dark colored. In places also there are fragments of various 

 modifications of the diorite inclosed in diorite, which appears to be a later 

 intrusion. 



The diorite also varies in mineral composition. For the most part the 

 ferromagnesian and the nonferromagnesian minerals are in about equal 

 proportions. In places the former preponderate. In other parts of the 

 rock the other minerals are in excess. The minerals recognized megascop- 

 ically are lime-soda feldspar, hornblende, and biotite. In places biotite is 



MON XXXII, PT II 7 



