ITS MICROSCOPIC CHARACTERS. 100 



position of the iron, become stained and present a rusty brown and gray 

 aspect closely simulating many mica schists. 



Owing to the production of dolomite, there results, in part at least, 

 ophicalcites and dolomitic limestones — the purity depending on the amount 

 of alteration, and on the materials both carried into and removed from the 

 reck during the process of alteration. These dolomitic limestones are usually 

 gray, green, or yellow, but sometimes of quite a clear grayish-white color, 

 and crystalline in structure. In the above, only a portion of the various 

 forms produced in the process of alteration so common in peridotic rocks 

 could be mentioned ; but it is hoped that enough has been given to afford 

 some idea of the general appearance of these rocks macroscopically. 



Of necessity, from the mode of origin of the peridotites and their ex- 

 posure to detrital agencies, various detrital or poroditic forms must result. 

 Undoubtedly, when they are re-consolidated, it is exceedingly difficult to 

 distinguish these true breccias, conglomerates, and sandstones from the 

 pseudo-fragmental forms of similar structure that are produced in this rock 

 species, as in every other, by the filling of fissures, or by the dissolving of 

 portions of the rock and their replacement by other material, while intersti- 

 tial portions of the rock remain in situ. The poroditic forms of the peridotites, 

 so far as now known, are all of a serpentinous character, having been 

 greatly altered ; but it is suspected, and in many cases claimed, that other 

 forms are of like detrital origin ; however, conclusive proof of this is still 

 wantino;. 



It is probable that part of our ophicalcites and brecciated serpentines are 

 of a poroditic origin, while others appear to have been produced by changes 

 in the massive rock i)i situ; that is, they do not properly belong to the 

 fragmental rocks to w^iich they are generally assigned. It is proposed here 

 to distinguish these falsely appearing detrital forms by the terms pseudo- 

 breccias, conglomerates, and sandstones ; or, collectively, as pseudo-frag- 

 mental or detrital rocks; or, better still, by the introduction of the term 

 merolite {fjiepo<;, XCOo^), and its adjective form mcrolitic, for them, since they 

 are composed of detached portions of the same rock. 



Section VI . — Peridotite. — Its Microscopic Characters. 



Beginning with the meteoric peridotites, the first variety is composed 

 of rounded fissured olivine grains with brownish glass inclusions, brown- 



