THE TERKE8TKIAL PHIilDOTlTES.— JjUXni:. 123 



found imbedded large masses of ])eridoLite. Tlie peiidotite is irregularly disseminated 

 through the serpentine, sliowing an intiniate connection of the two. The peridotite is 

 found to be composed of olivine grains, traversed by numerous fissures, and containing; 

 irregular fragments and octahedrons of picotite. The rock itself usually varied from a 

 greenish-gray to various shades of green. 



The serpentine is generally of a dark-green color, traversed frequently by veins of 

 clirysotile, and not uncommonly charged with crystals of diallage. It is said that in some 

 places the serpentine " is traversed by great parallel planes of fracture, which at first sight 

 might be mistaken for stratification," 



The alteration of the olivine takes place along the fissures, the iron sef)arating in the 

 serpentine as magnetite and chromite. This serpentinizatiou gradually extends until 

 only small grains of olivine are left, and then on until the entire rock is altered to serpen- 

 tine. A perfect and gradual transition was traced from the beginning of the process to 

 the complete transformation. The alterations are shown very well in the figures accom- 

 panying Macpberson's paper. 



Serrania cle Ronda, Spain. 



This rock, according to Macpherson, is of a clear, greenish-gray color, with a lustre 

 between a greasy and vitreous. The section is composed of a crystalline-granular aggre- 

 gate of olivine ftagmeuts, containing numerous picotite grains. The olivine shows brilliant 

 polarization colors, and sometimes a striation parallel to the plane of extinction, while it 

 is traversed by irregular fissures.* 



St. PauVs Hocks. 



These rocks were described by Darwin as unlike any rock he had met. He states : 

 " The simplest, and one of the most abundant kinds, is a very compact, heavy, greenish- 

 black rock, having an angular, irregular fracture. . . . This variety passes into others of 

 paler-green tints, less hard, but with a more crystalline fracture. . . . Several other varie- 

 ties are chiefly characterized by containing innumerable threads of dark-green serpentine, 

 and by having calcareous matter in their interstices. These rocks have an obscure, con- 

 cretionary structure, and are full of variously colored angular pseudo-fragments. . . . 

 There are other vesicular, calcareo-ferruginous, soft stones. There is no distinct strati- 

 fication, but parts are imperfectly laminated, and tlie whole abounds with innumerable 

 veins, and vein-like masses, both small and large," Darwin states that the rock is not 

 of volcanic origin — not necessarily meaning by this anything more than that it was 

 not a modern eruptive formation like that of the other islands visited.f 



These rocks being the haunts of birds, a phospatic incrustation had been formed on 

 part of the surface, and Professor Wyville Thomson states " that they look more like the 

 serpentinous rocks of Cornwall or Ayrshire, but from these even they differ greatly in 

 character. . , . Mr. Buchanan is inclined to regard all the rocks as referable to the ser- 

 pentine group. So peculiar, however, is the appearance which it presents, and so com- 

 pletely and uniformly does the phosphatic crust pass into the substance of the stone that 

 I felt it difficult to dismiss the idea that the whole of the crust of rock now above water 

 might be nothing more than the result of the accumulation, through untold ages, of the 



* Anal, Soc. Esp., Hist Nat., 1879, viii. 251, 252. 

 t Volcanic Islands, 1851, pp. 31-33, 125. 



