164 PERIDOTITE. 



blended with, and entirely lost in, the serpentine groundmass. Oftentimes 

 the pyroxene minerals are replaced by greenish to whitish talcose material 

 and talc scales. In the process of alteration, segregations of serpentine, 

 dolomite, magnetite, chromite, etc., occur, giving rise to veins of serpentine 

 (chrysotile) and to veins and nodular masses of dolomite and iron ores lying 

 in or traversing the serpentinous groundmass. 



However dark the altered peridotites may be in color, the thin splinters, 

 as a rule, are translucent and transmit a greenish or yellowish light. The 

 more or less serpentinized peridotites are traversed by fissures, which are 

 most abundant in those entirely changed to serpentine. The sides of 

 these fissures usually are polished or coated with serpentine, talc, etc., 

 forming " slickensides " ; which, it is conceived, may have some connection 

 with the chemical alteratiou of the rock itself. 



Amongst the various forms produced by the extreme changes are a 

 yellowish, more or le.ss gummy-looking substance, and a grayish, yellowish, 

 to chrome-green translucent serpentine. While these are oftentimes pro- 

 duced from the alteration of the rock in situ, they also appear to be formed 

 by migrated serpentinous material, and in such cases to belong to the vein- 

 stones. These secondary massive products contain more or less iron ore 

 in the form of chromite or magnetite. It is to these nearly pure serpen- 

 tines, which result from the complete change or migration of the material, 

 that the term serpentine, as it is used in works on mineralogy, properly applies. 

 But from the general and microscopic characters of the material known as 

 serpentine in lithology, it would appear that under that name is placed a 

 mineral of variable composition, forming a series like feldspar or pyroxene, 

 or else .several distinct minerals are now so placed. 



Further, in the process of alteration there often results a fissile or schis- 

 tose structure, giving rise to a pseudo-lamination. In part this seems to be 

 owing to the segregation of chrysotile, serpentine, iron ores, dolomite, etc., 

 in approximately parallel lines ; and in this case the fissility is often only 

 apparent and not real. Sometimes the schistosity seems to be due to pres- 

 sure during the time of alteration. Occasionally the rock has a brecciated or 

 conglomerate structure, owing to the vein serpentine or dolomite surround- 

 ing less altered portions of the rock. With the formation of talc or actinolite 

 in these altered peridotites, the transition to a true schist is evinced by 

 various gradations, until a true talc schist or actinolite schist results. These 

 schists are greenish in their normal condition, but often through decom- 



