518 Noble — Geology of the Grand Canyon, Arizona. 



thinned by a strike fault, precluding the possibility of measur- 

 ing the thickness in that direction. 



Petrography. 



Megascopic. — The fresh specimens characteristic of the 

 greater part of the mass show it to be a tough, heavy, non- 

 crystalline rock of medium to coarse grain and of a grey color. 

 The minerals visible to the unaided eye are plagioclase, olivine, 

 augite, and an occasional grain of magnetite. Although the 

 olivine exceeds the augite in amount, it is less conspicuous to 

 the eye. Aside from a somewhat waxy luster of the feldspars, 

 the rock is remarkably fresh. The weathered surface has a 

 characteristic warty appearance, imparted by the presence of 

 lumps or balls which are of a coarser grain and different tex- 

 ture than that of the mass of the rock and are more resistant 

 to the processes of disintegration. These lumps and balls can 

 be seen to consist of coarse ophitic intergrowths of augite and 

 plagioclase. The diabase weathers by mechanical disintegra- 

 tion to a greenish-olive sand in which are innumerable lumpy 

 kernels of all sizes derived from the ophitic masses described 

 above. The rock has no typical columnar structure, but usu- 

 ally displays a rough vertical jointing such as is characteristic 

 of granite. 



Microscopic. — The slides examined show the typical rock to 

 consist primarily of plagioclase feldspar (near labradorite) and 

 olivine in about equal amounts, with a subordinate quantity of 

 augite and brown biotite. A very little magnetite is present. 

 The feldspar is somewhat altered, but all the other minerals 

 are fresh. The olivine occurs characteristically in rather large, 

 rounded crystals of automorphic habit. The augite is chiefly 

 confined to the globular masses which weather out as lumps 

 and kernels and does not characterize the rock as a whole. 

 Slides cut from these kernels show them to be composed 

 entirely of augite and feldspar. The augite is enclosed within 

 the feldspar, displaying beautiful examples of ophitic texture. 

 Several of the magnetite crystals were observed to have rims 

 of brown biotite. The small amount of magnetite is rather 

 remarkable, and it seems likely on this account that the olivine 

 is rich in magnesia. Because of the predominance of olivine 

 and plagioclase in the greater part of the rock, the diabase is 

 classified as an olivine-diabase with a troctolitic aspect. 



Variations in character. — All parts of the mass are subject 

 to variations in texture and composition toward a coarser grain. 

 These are of two kinds. The first type occurs in the ophitic 

 intergrowths of augite and plagioclase in the lumps and balls 

 described above, and is a segregation phenomenon character- 



