132 PERIDOTITE. 



and alteration-products represented by the gray and brown lines running from top to 

 bottom. The enstatite crystal is crossed from right to left by a yellowish serpentine vein 

 connecting two portions of the altered olivine mass represented by the mixed brown, 

 yellow, and white. The dark grains are the iron ores, or picotite. 



Figure 1, Plate VII., represents an enstatite crystal from the same rock, in a more 

 highly altered condition. The primary cleavage runs from right to left, and the secondary 

 from top to bottom. The greenish color shows the earlier stages of the alteration, and 

 the yellow color the following or serpentine stage, although part of the serpentine mate- 

 rial may have been brought in from the surrounding olivine mass not shown in the 

 figure. 



Foot of divide between Round Valley and Bullfrog Creek, Inyo Co., Cul. 



3003. A compact oil-green groundraass, holding bluish-black and bronze-like crys- 

 tals of enstatite. The rock is traversed by a few veins of pale-green serpentine, while 

 a chrysotile vein occurs at one end. 



Section : a yellowish-green groundmass, liolding porphyritically enclosed some dark 

 enstatite plates. The groundmass shows under the microscope the same reticulated net- 

 work of veins as tliat seen in 3001 and 3002, the veins being readily distinguisliable 

 both in common and polarized light. While the structure remains in general the same as 

 in 3001 and 3002 the entire groundmass is changed to serpentine : that is, it is a dis- 

 tinct pseudomorphous replacement by serpentine of all the essential structural character- 

 istics of the preceding rocks. Even black opaque masses are seen having all the structural 

 features of the picotite of Nos. 3001 and 3002, 



Much magnetite or chromite is seen scattered throughout the groundmass, or collected 

 into open aggregations. The enstatite in some cases retains its usual polarization charac- 

 ters, with the well-marked cleavage. In others it has been so highly altered that only 

 traces of the cleavage and the orthorhombic extinction in polarized light remain of its 

 usual diagnostic features. All the enstatites are filled with grains of magnetite, which in 

 some crystals are arranged along the cleavage lines. Tlie powder of the enstatite crystals is 

 magnetic, and it is to the magnetite that their bluish-black color is due. The magnetite 

 is regarded as a product formed during the conversion of the rock into serpentine. The 

 evidence afforded by the microscopic structure of this rock, it seems to me, is proof posi- 

 tive that this serpentine was formed from the metamorphism of a peridotite, the beginning 

 of which change is to be seen in No. 3001, and still further advanced in No. 3002. 



The general structure of this section is shown in figure 4, Plate VI., which displays 

 an altered enstatite crystal with its secondary magnetite, surrounded by the serpentine 

 replacing the olivine. 



Mohsdorf, Saxony. 



This rock, according to Dr. E. Dathe, is compact, blackish-green, and contains crystals 

 of diallage and enstatite, which show a mother-of-pearl to silky lustre, and a light-yel- 

 lowish color, while they are finely striated. The principal material of the section is 

 olivine, part of which is in large rounded grains with few fissures, and part in smaller 

 grains, traversed by cracks, and more or less altered to a greenish-fibrous serpentine. 

 The olivine contains little octahedral crystals of chromite or picotite, witli rounded angles. 

 The enstatite is in light-greenish, elongated sections, sometimes holding olivine grains, and 

 traversed by cleavage lines parallel to 010. Diallage also occurs, recognized by its opti- 



