192 PERIDOTITE. 



peridotites, especially those contiiiniiig pyroxene minerals; but the present 

 writer's studies woidd indicate to him that the most common source that 

 yields by alteration our soapstone or steatite rocks, is to be sought in our 

 gabbros and coarser crystalline diabases (diorites). The purer talcose rocks 

 would appear to come from the former source (the peridotites), the more 

 impure ones from the latter (the basalts). 



From the evidence in the preceding pages, it is probable that some actino- 

 litic and other schists result from the alteration of the peridotites, although 

 in general the amphibole schists appear to belong to other groups. 



The formation of impure dolomites from peridotites would seem to have 

 been clearly shown in the preceding pages, but how far this will account for 

 the common association of magnesian limestones with serpentine, is a problem 

 for the future. One thing, however, appears clear, that such limestones are 

 produced on a small scale, and sometimes on a more extended one in connec- 

 tion with the general alteration of peridotites into serpentine. 



It is not the part or intention of the writer to explain the modes of 

 change in these rocks. It is rather his part to give the facts observed, and 

 for the mineral chemist to engage in the work of explanation, unless he can 

 impeach the facts presented. 



Section X. — Peridotite. — Its Classification. 



As previously stated [ante, pp. 84, 85), all rocks of this class are here 

 grouped under one species or type — peridotite ; while for the modifications 

 produced by the variation in mineral composition, varietal names are em- 

 ployed, in deference to the views of those who make species out of every 

 mineral variation in rocks. Since, so far as practicable, these variety names 

 are the same as the specific names of other lithologists, and have the same 

 general limits, no difficulty will arise in their use, whether the person em- 

 ploying them looks upon each division as a specific or a varietal term. 



In accordance with the methods of this work, ijeridotite is defined as 

 including all meteoric and terrestrial rocks of every age, which are composed 

 essentially of olivine, with or without pja'oxene minerals, and iron ores 

 including picotite. 



It has not been customary to base any varietal distinction on the iron 

 ores, or hardly to look upon them as essential, although they are universally 



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