MUSFX'M OF COMPARATIVE ZOuLOfJY. 61 



of Ihcrzolite. In places, this rock is much altered, forming a serpentine 

 (65, G6, G7, G8, 69, 70, 71, 72, 73, 74). We find every gradation, from 

 the rock only partly altered to that which is so completely changed to 

 serpentine that only traces of the cnstatite, diallage, and olivine remain. 



Under the microscope, the rock (65) is seen to be made up of rounded 

 grains and crystals of olivine, held in and often completely surrounded 

 by the enstatite and diallage. These minerals evidently crystallized later 

 than the olivine, and play the same rule here that the augite does in dia- 

 base, the glass in basalt, and quartz in granite. The olivine is traversed 

 by fissures, along which the usual serpentinous alteration has taken place. 

 In many, the alteration is confined to the vicinity of the fissures and the 

 periphery of the olivine, but others are changed throughout. Much 

 black dust comes in the altered portions (magnetite ?), as a residue 

 left over in the deconjposition of the olivine and the formation of the 

 serpentine. In many cases this black residuum forms a rectangular or 

 irregular network or grating throughout the changed olivine. The 

 enstatite is altered along the cleavage planes and network of fissures 

 by which it is traversed. It does not become changed to the serpentine 

 so readily as the olivine. All contain inclusions of black octahedral' 

 crystals that are presumably magnetite, as the powder is magnetic, and 

 no trace of chromic oxide was found by chemical tests. Besides the ser- 

 pentine, there occur, as other alteration products, feldspar (1), viridite, 

 and dolomite (]). The hand specimen (65) is a grayish black (almost 

 black) rock, showing under the glass a little serpentine, enstatite, and 

 magnetite. It weathers somewhat brownish. 



In other specimens from the same rock, but more altered, we only 

 find traces of the original structure. The formation of the serpentine 

 along the fissures, and the network of magnetite, usually are well marked 

 after the enstatite, diallage, and olivine are entirely altered. The serpen- 

 tine, unless it suffer alteration itself, generally shows under the microscope 

 its original structure, as it was formed along the fissures. One section 

 (71) shows simply greenish pseudomorphs after olivine enclosed in a car- 

 bonate, presumably dolomite. Another section (74:) is composed now of 

 serpentine and magnetite, and, if it were not absolutely known whence 

 it came, its derivation could only be told by the arrangement of the mag- 

 netite. Certain of the specimens are largely composed of the dolomite 

 observed in No. 71, and microscopic as well as field examination renders 

 it most probable that Dr. Rominger's stratified dolomite (72) is simply 

 a more highly altered portion of the peridotite. Xo. 71 came from the 

 east side and out of the same upper formation as Xo. 72. This is a 



