840 DESCRIPTIVE GEOLOGY. 



dark-brown rock, in which are no recognizable crystals, up to a coarse- 

 grained mass, through which decomposed crystals of feldspar are very thickly 

 scattered. The prevalent color is dark green, and in general the texture 

 rather earthy. Under the microscope. Professor Zirkel found them to consist 

 of tri clinic feldspars, decomposed augite, a little hornblende, and considerable 

 olivine. Many of the outcrops at the head of the cafion, near the summit of 

 the melaphyr body, are characterized by green spots the size of a pea, partly 

 formed of delessite, and partly of well-crystallized calcite. Nearly all the 

 feldspars in the upper portion are more or less converted into carbonate of 

 lime. There are present impure crystals of carbonate of lime half an inch 

 long, rendered opaque by green earth and some small, gray grains, whose 

 nature is not known, probably referable to augite. 



A little lower down the canon occurs a chocolate-brown variety, in which 

 the feldspars are mainly converted into carbonate of lime, but which is charac- 

 terized by spherical amj^gdules from one-half inch downward until they are 

 too small to be visible. The surface of these amygdules has a pitted appear- 

 ance, like the smooth interior of certain empty cavities in basalt. They were 

 evidently, in the first place, filled with green earth, which has been subse- 

 quently more or less converted into calcite. Only a few feldspars are still 

 preserved in anything like their original condition, and under the microscope 

 these all appear to be triclinic. About a mile down from the head of the 

 canon is the most altered zone in the melaphyr body. Here the entire 

 mass is decomposed, and apparently consists of delessite and carbonate of 

 lime, while the amygdules no longer show their spherical form, but are 

 broken and run together until the rock is little more than a network of car- 

 bonate of lime, rendered impure by streaks and grains of delessite, with a 

 general filling of impure delessitic matter, in which are the faint traces of 

 feldspar, and brown earthy material, probably representing the augites. In 

 this zone are found also large siliceous amygdules, which are in fact quartz 

 geodes surrounded by a variable coating of carbonate of lime and green 

 earth. In some instances, in the hollows of the calcareous amygdules, is a 

 secondary growth of quartz crystals. 



Descending the canon, the first rock which is found to break through 

 and overlie the melaphyrs is a pinkish-gray body, colored on the geologi- 



