ITS OEIGIN. 189 



to be present, and in the serpentine rocks it appears in general to vary from 

 8 to 16 per cent. 



In general, the peridotites are relatively low in their percentages of silica, 

 alumina, and lime ; high in magnesia ; and variable in iron and specific 

 gravity ; the unaltered forms being higher, in both cases, than the altered. 



Section IX. — Peridotite. — Its Origin. 



"Are the peridotites sedimentary or eruptive ? " is an exceedingly impor- 

 tant question, and one which, in the case of serpentine, has been frequently 

 argued. 



That the peridotites are sometimes eruptive, the cases cited by Bonney, 

 Diller, and the present writer, for Cornwall, the Troad, and Lake Superior, 

 are clear, explicit, and decisive — that they occur in dikes, in intrusive 

 tongues, and in uplifting, altering, eruptive bosses, showing the same char- 

 acters as do other eruptive rocks, especially those of a coarse-grained charac- 

 ter like gabbro and granite. That the peridotites are in part eruptive we 

 should, then, consider a settled fact ; but another case still remains to be 

 considered : the association of these I'ocks with schists. 



Where schistose rocks are associated with the peridotites, especially the 

 serpentine variety, their relations would appear to be accounted for in the 

 following ways : — 



1. They may occur as eruptive rocks which, with their associated country 

 rocks, have later been altered, producing a general apparent blending of all 

 into a single series ; as it is well known has taken place with the older erup- 

 tive basaltic rocks — diorite, diabase, melaphyr — which lias caused them 

 also to be looked upon as interbedded sedimentary rocks, in common with 

 their associated schists. 



2. All may have come from the alteration of eruptive material, — the len- 

 ticular patches being the remnants of the altered rock. This would be in 

 accordance with the well-known mode of alteration of olivine rocks, in which 

 occur clear grains of unaltered olivine surrounded by the serpentine and 

 other secondary minerals which often give to the rock a schistose character. 

 We might as well claim that these clear olivine grains were produced by the 

 metamorphosis of the serpentine and schistose material, when ,the reverse is 

 known to be the case, as to claim that the patches of olivine rocks in schists 



