ITS CLASSIFICATIOX. 103 



present, or nearly so. The varietal distinctions (specific, of other lithologists) 

 are founded on the pyroxene minerals and on the alteration-products. Thus 

 diinite is the term given to the form of pcridotite which is essentially com- 

 posed of olivine ; mxonitc to that composed of olivine and enstatite ; Ihn-zolite 

 to that composed of olivine, enstatite, and diallage ; ImchnerUe to that com- 

 posed of oHvine, enstatite, and augite : cnlysite to that composed of olivine 

 and diallage ; and picrite to that composed of olivine and augite. 



Under these varieties are classed all the forms produced by alteration, so 

 fiir as they may retain sufficient original characters for their identification ; 

 when they do not, then they are placed under a variety name belonging to 

 the produced form. Thus, serpentine is given as the variety name for all the 

 altered peridotites in which serpentine forms the essential constituent; /ak- 

 schist to all in which talc holds a similar part ; and in the same way any 

 variety produced by alteration can be designated by its common name when 

 its derivation is known. 



It is, however, expected that future studies will lead to the discovery 

 of every mineralogical combination that can be formed by the principal 

 silicate constituents of peridotite. These combinations, so far as now 

 known grade into one another, and it is to be expected that all other dis- 

 covered ones will do the same. Hence, if the ordinary methods of nomen- 

 clature should be followed, the number of species or varieties of peridotite 

 W'ould be great and their separation difficult. However, the present writer, 

 as he has previously stated, does not place any stress upon the subdivisions 

 now made on a mineralogical basis in peridotite, but adopts them as a con- 

 venience only in the present state of lithological science ; and they can 

 readily be replaced by the general emploj^ment of the specific name — 

 peridotite. 



The Umhiirgite of Rosenbusch has not been placed here with the peridotites, 

 for its microscopic structure is like that of some of the porphyritic glassy 

 basalts, and differs much from the peridotites. While its percentage of silica 

 is like that of this species, its contents of alumina and lime ally it to a more 

 acidic group ; although it is not improbable that it belongs to the picrite 

 variety. Except in its abundant olivine, limburgite microscopically closely 

 approaches some of the andesites ; but, taking its characters as a whole, 

 the majority of them appear to be basaltic, and with that group it will here 

 be classed until further evidence can be procured. 



The fragmental states of the peridotites are indicated for the unaltered 



