PETROLOGY 315 



sions of light minerals, however, are not composed entirely of the feld- 

 spars, since some nepheline occurs in them protected by a network of 

 small feldspar crystals. 



Microscopic character. — The mineralogical composition of the two 

 nepheline syenite dikes is essentially the same except that in the north 

 dike the chief mafic mineral is biotite, while in the south dike it is horn- 

 blende. In both instances the dark minerals constitute only a small per- 

 centage of the rock. The rock consists essentially of the felsic minerals 

 feldspar (orthoclase and albite) and nepheline, which in the thin-sections 

 appear in nearly equal amounts, although the relative proportions vary 

 greatly in different parts of the same section. 



The dark constituents, named in order of abundance, are hornblende, 

 biotite, and iron oxide (probably magnetite). An occasional crystal of 

 augite is sometimes developed in the rock of the north dike. Apatite and 

 rutile occur as minor microscopic accessories in the form of inclusions in 

 the other minerals. Alteration of the original minerals has resulted in 

 the development of abundant zeolites, kaolinite, calcite, chlorite, musco- 

 vite, epidote, and iron oxide. 



The two feldspars, orthoclase and albite, appear to be present in about 

 equal quantity, but are unevenly distributed through the sections. They 

 (especially plagioclase) are very abundant in some parts of the sections 

 and almost absent from others. Albite occurs in idiomorphic latn-shaped 

 forms and in allotriomorphic grains and exhibits characteristic multiple 

 twinning. It was one of the earliest of the essential minerals of the rock 

 to crystallize, and in those areas of the thin-sections where it is especially 

 abundant the lath-shaped forms cutting the other minerals give the rock 

 a pronounced ophitic texture. It is usually clouded with dustlike . Delu- 

 sions. In the north dike, which represents the porphyritic type of the 

 nepheline syenite, orthoclase occurs in two distinct forms, as Idiomorphic 

 forms developed as phenocrysts and as allotriomorphic grains in the 

 ground-mass. In the south dike it occurs only as formless grains, ex- 

 hibiting good cleavage development in two directions. Except horn- 

 blende, orthoclase is the freshest mineral in the rook. 



Nepheline is an- abundant constituent in all the thin-sections; it occurs 

 both as crystals showing rectangular and hexagonal outlines and as 

 masses of irregular outline. Its period of crystallization was earlier than 

 that of the mafic silicate minerals and it frequently cuts or is included 

 id them. The nepheline is usually more or less altered, a verv common 

 form of alteration being to fibrous zeolites, whose fibers are' arranged 

 Qomal to the altered surface. More or less calcite and other secondary 

 products in small particles are often associated with (he zeolites, and 

 XXii: — Bull. G»ol, Soc, Am., Vol. 24, ji»rj 



