190 PEEIDOTITE. 



■were produced by the metamori^hosis of the schistose material, when the 

 reverse is probably true. 



3. All other eruptive rocks are subjected to denudation, and their debris 

 piled up somewhere with other associated sedimentary beds ; hence there is 

 every reason to believe that we have detrital peridotitic rocks, the same as 

 we have detrital basaltic ones. 



All three of the preceding methods are probably correct for some cases, 

 but which method is to be advocated in each case is to be determined by the 

 study of the locality in question. 



Although as yet it does not seem to be proved that peridotic volcanoes 

 have existed, and peridotic ash may not have been formed, we should look 

 for all the intermingling of the eruptive peridotic material with its own debris 

 and with that of other rocks, the same as previously set forth in general 

 terms for rocks of this origin {cmte, pp. 22-24). We should then expect to 

 find the peridotites occurring in dikes and eruptive masses, in altered schis- 

 tose or foliated rocks, in detrital beds, and in every form and every associa- 

 tion that it is possible for eruptive rocks and their debris to exist in. The 

 study of these occurrences would, however, be expected to be just so much 

 more obscure than is the study of the basaltic rocks, as the former are more 

 basic and easily alterable than the latter. 



That serpentines are produced from the alteration of peridotic rocks, is 

 the testimony of all lithologists who have studied their structure with the 

 microscope, and it is one of the most fixed facts in the science. That serpen- 

 tines are not produced in other ways may perhaps be looked upon as an open 

 question at the present time, although there would seem to be no proof of 

 any other mode of origin. In the case of any mineral formed by secondary 

 changes, more or less migration of the mineral material is apt to occur, 

 and in this way the secondary serpentine is frequently found in veins, 

 and in other localities outside of the rock from whose alteration it was 

 produced. 



That serpentines have come from chemical precipitates from ocean waters, 

 is a view which certainly does not appear to have any proof in its behalf, and 

 one which probably arose from the confounding of veinstone-serpentine or 

 migrated serpentine material, with that produced by the alteration of perido- 

 tites in situ. The production of serpentine by the alteration of olivine and 

 its removal and storage in fissures and adjacent rocks, brings in connection 

 with the peridotites the difficult problems belonging both to eruptive rocks 



