DIKE ROCKS OF ELECTRIC PEAK. 95 



gradual variation in the kinds of lime-soda feldspar accompanying that of 

 the ferromagnesian minerals. But transitions in the possible isomorphous 

 series of hornblendes or pyroxenes were not detected. The variation in min- 

 eral composition affects the microstructure of the grountlmass, an increase 

 of quartz being accompanied by an approach to a granular structure. 



Although the coarseness of grain and at the same time the character 

 of the microstructure vary to some extent with the size of the dike, and to 

 a greater extent with the rate at which the mass cooled, for each of the 

 mineralogical subdivisions indicated in the table, still, the conditions under 

 which these various magmas cooled having been quite similar, the variations 

 in the composition and microstructure of the groundmasses of the subdivi- 

 sions named bear a marked relation to the composition of the rock as a 

 whole and to the variations in the phenocrysts. This will appear from the 

 following descriptions of the rocks belonging to the subdivisions in Table I: 



(a) Groundmass with niicropoikilitic structure, the quartz grains iu one case being 0.09 mm. to 0.43 

 mm. in diameter, and containing lath-shaped microlites of feldspar, gas cavities, and microscopic 

 hornblendes and biotites. Abundant phenocrysts of lime-soda feldspar, 1 to 2 mm. long, and 

 smaller ones of hornblende and biotite. The relative proportions of the latter minerals differ 

 in different specimens. As the total amount of ferromagnesian minerals increases, the relative 

 amount of hornblende increases. Small amounts of magnetite, apatite, and zircon occur in 

 all these rocks. 



(ft) The rocks of the second subdivision are nearly the same as those of the first. 



(c) In the third division the micropoikilitic structure is not well marked and grades into one in 

 which lath-shaped feldspar microlites are more prominent. 



(fZ) and (e) Micropoikilitic structure is wanting. The groundmass is composed essentially of lath- 

 shaped microlites and grains of feldspar, approaching a fell like or pilotaxitic structure. This 

 change accompanies a diminution iu the amount of quartz. In these five subdivisions pyroxene 

 is entirely absent. 



(/) and (</) These are similar to the last two subdivisions in microstructure and composition, but 

 pyroxene was originally present as phenocrysts and has been uralitized. Biotite is only 

 present in shreds in the groundmass and does not form phenocrysts. 



(ft) is represented by a coarser-grained rock, but slightly porphyritic. It consists of lath-shaped 

 feldspars 0.4 mm. to 0.7 mm. long, between which is a very small amount of irregular grains of 

 feldspar and quartz and ferromagnesian silicates, amphibole, and mica. There is much 

 uralitized pyroxene, which formed the largest idioiuorphic crystals. 



(i) The rock of this division is like the last in microstructure, but is more porphyritic, with pheno- 

 crysts of plagioclase and pyroxene, the latter in part uralitized. Near the contact of this rock 

 with metamorphosed sandstone the pyroxene is almost colorless, different from the varieties 

 common in the pyroxene rocks of this region. It resembles the secondary pyroxene whicli has 

 resulted from the alteration of hornblende in other varieties of porphyry in this neighborhood. 



The more quartzose porphyries and the coarse-grained modifications 

 cutting the stock will be described later. The microscopical characters of 

 the minerals constituting the dike rocks are nearly the same throughout 

 this series of rocks. 



