638 STOSE AND LEWIS TRIASSIC IGNEOUS ROCKS 



alters to serpentine or chlorite. The pyroxene contains but little alumina, 

 and that required for chlorite is undoubtedly derived in the main from 

 accompanying feldspars. This is further shown by the fact that scales of 

 chlorite have developed, in the feldspars, as well as in minute veinlets that 

 intersect all the minerals, proving the migration of its constituents. The 

 secondary minerals after pyroxene are commonly darkened by granules, 

 and in some places trellis-like skeleton crystals, of magnetite. Locally 

 these are so abundant as to blacken the whole space occupied by the origi- 

 nal pyroxene. Calcite is rare among the decomposition products, even in 

 the most altered specimens. 



Jlypersthene. — With the exception of pleochroism and parallel extinc- 

 tion, hypersthene has the general appearance and characteristics of the 

 monoclinic pyroxene. It alters to uralitic amphibole and serpentine, al- 

 though, as a rule, it is fresher than the monoclinic pyroxene. 



Plagioclase feldspar. — Generally the plagioclase forms the well known 

 lath-shaped or rod-shaped crystals, but in some of the rock of coarser 

 texture, with much quartz and orthoclase, it is much less elongated and the 

 texture approaches the granitic. Terminal planes are commonly lacking 

 and twinning lamellae are universal, commonly according to the albite 

 law, although pericline and carlsbad twinning are also observed. Zonal 

 structure is rather common and, fringing the extreme acid borders, a 

 graphic intergrowth of orthoclase and quartz generally fills some of the 

 smaller interstices. Extinction angles correspond generally to an ande- 

 sine-labradorite. Apatite, magnetite, and olivine occur as inclusions. 

 The plagioclase most in evidence in the microscopic study seems to be 

 largely andesine-labradorite, but chemical analyses of similar material 

 from other localities shows the presence of a much wider range of Jeld- 

 spars. 



Alteration gives rise chiefly to a confused fine scaly aggregate with 

 high double refraction, apparently sericite (paragonite?), with a little 

 kaolin in places. With migration of iron and magnesia from the' py- 

 roxene, chlorite is developed, as described above. 



Orthoclase, microcline, quartz. — Quartz and orthoclase in micrograph! c 

 intergrowth are very common, but separate grains large enough to be seen 

 in the hand specimen are generally present in the lighter colored coarse- 

 grained varieties of the rock. In these, areas of micropegmatite 3 or 4 

 ' millimeters in diameter are so abundant in places as to constitute three- 

 fourths of the rock or more. Microcline is comparatively rare. Orthoclase 

 is commonly much more altered than plagioclase — generally to a chalky 

 kaolin-like substance, but in places to sericitic aggregates. 



