DERIVATION OF SERPENTINE FROM PYROXENITE. 407 



intermediate between the shale and serpentine : First, what would be the 

 effect of those agents upon the shale which produced the conversion of the 

 original rock of the dike into serpentine? The original rock may have 

 been a pyroxeuite, of which there are examples in the dike at some distance 

 from the locality of the specimens under discussion, and an analysis of which 

 is given under /. The large amount of iron in serpentines in general con- 

 firms the supposition of derivation of this class of rocks from ferro-mag- 

 nesian silicates. The ferro-magnesian silicates are provided for in the bron- 

 zite and dillage of the pyroxenite, the latter constituent carrying consider- 

 able calcium oxide. The leaching out of considerable silica and almost all 

 of the alumina and lime from the pyroxenite /by hot waters, which at the 

 same time hydrate the residue, yields the serpentine d. Now all mineral 

 waters contain a quantity of magnesia in solution, so that it might be as- 

 sumed that some of the magnesia of the pyroxenite goes into solution. In 

 this way, then, a transfer of magnesia to a portion of the shale h adjacent 

 to d may be effected, w^hile about 10 per cent, of silica is removed, as in the 

 case of pyroxenite. The hydration goes on at this point also, by which 

 about 4 per cent, water is added. The shale along the contact is thus con- 

 verted into a more highly magnesian rock. The explanation of the loss of 

 alkalies in c is found in the easy kaolinization of the feldspathic constituents 

 of the shale under the assumed conditions. The high percentage of alumina 

 shows without question that specimen c was originally shale. Hence it 

 would seem that the process of metamorphism penetrated some distance into 

 the shale before being arrested. 



The second supposition, namely, that surface waters brought about the 

 change in chemical composition along the contact of shale with the already- 

 formed serpentine, seems to me not to carry with it that conviction which 

 the first supposition does. For instance, the leaching out of about 10 per 

 cent, of silica from h to yield that per centage in c would hardly take place 

 undert he ordinary conditions wdiich 'sve witness to-day. 



In precisely the same way as the serpentine (number 181), the bastite e 

 (number 176), which possesses very nearly the same composition as the ser- 

 pentine, can be regarded as produced from pyroxenite. 



Series III. — The specimens of this series were found on the western slope 

 of the mountain in a branch of the Arroyo del Cerro. The serpentines are 

 from the dike described in Mr. Turner's notes, and the shales occur at the 

 exact contact with this dike. 



a. Slate-colored shale (1) in large lumps, friable, with a few very narrow 

 and loose seams of a white substance marked a (2), which were all but re- 

 moved from the shale before analysis. The composition both of the shale 

 and of the material of the seams was determined by separate analyses. The 



LX— Bui.i,. Geot,. Soc. Am , Voi,. 2, 1890. 



