118 PROCEEDINGS OF THE ACADEMY OF [1890. 



hardness of the serpentines, and by the rarity of minerals, except 

 varieties of serpentine, and asbestus, enstatite and quartz, the latter 

 resulting from the alteration of the serpentine. At nearly all the 

 outcrops there is evidence, and at some of them almost absolute 

 proof, that the serpentine results from the alteration of the enstatite. 

 Chromite, Genthite and Ripidolite occur rarely. 



The pseudomorphism of enstatite into serpentine may be most 

 clearly seen at Rose's quarry on the La Fayette belt about one thou- 

 sand feet west of the Schuylkill, where the same stratum is enstatite 

 below, serpentine above, but rocks in process of change may be found 

 at many of the other outcrops, especially along the great Radnor 

 belt, also at the outcrop S. W. of Newtown Square and where 

 the LaFayette belt crosses Darby Creek near the southwest corner 

 of Radnor Township. 



It is true that Dr. T. Sterry Hunt ' contends that the origin of the 

 serpentine and related magnesian rocks was to be found in deposits 

 of hydrous silicates like the magnesian marls of the Paris basin and 

 that the enstatite, etc. are derived from the serpentine. 



In the region under discussion, the evidence is that the enstatite 

 and the serpentine are pseudomorplious one of the other. Inas- 

 much as there frequently occur serpentine pseudomorphs after other 

 minerals, in which the crystalline form of the original mineral is pres- 

 ent, e. g., after chrysolite, enstatite, staurolite, pyroxene, hornblende, 

 etc., while there is no evidence whatever that any of these minerals, 

 or probably any others, except quartz, have resulted from the decom- 

 position of serpentine, I incline to the other view. This is supported 

 too by the mode of occurrence. The enstatite at Rose's quarry on 

 the Schuylkill opposite LaFayette station is found at the bottom ; as 

 the stratum rises it can be seen to be more and more changed grad- 

 ually until, near the top, true serpentine results and the enstatite 

 disappears. There is, throughout, evidence of ^reat pressure in a 

 slaty structure and slickensides, proof that the forming mineral occu- 

 pies the greater bulk, which would be the case if serpentine is the re- 

 sulting mineral. Enstatite being a crystalline mineral would it not, 

 if formed by dehydration from a hydrous magnesian silicate change 

 abruptly into more or less crystalline masses rather than in the uni- 

 formly progressive mode in Avhich we find it, whereas the latter is 

 exactly as we should expect to find hydration occur. 



1 Min. Phys. & Phys., p. 432. 



