﻿168 PKOF. T. G. BONNEY ON ANTIGOEITE [^^-y IQoS, 



like habit of antigorite. The third consists of a mass of acicular 

 flakes with a general parallelism and slightly wavy structure, a few 

 of which have a higher refractive index than the rest. This 

 approaches nearer to the antigorite exhibited in very fissile speci- 

 mens, but cannot be called a satisfactory examxjle of that mineral. 



Thus the evidence of these specimens of crushed bastite-serpen- 

 tines from the Rhsetian Alps seems at first sight unfavourable to 

 the very clear and precise statements of Becke and Preiswerk. We 

 may perhaps offer the following explanation of the discrepancy. 

 When a peridotite rich in olivine (for instance, dunite) is acted on 

 by pressure,^ it becomes slightly slabby, and microscopic examination 

 shows that mineral to occur in grains, rather variable in size, with 

 some little approach to a parallel ordering ; in other words, the 

 rock shows signs of a crush-structure. Turning to the antigorite- 

 rocks of the Alps, we find that some of these exhibit, by means of 

 the residual augite, clear indications of having been more or less 

 crushed. The flakes of the antigorite in some cases are arranged 

 at right angles to the pressure, but in others they lie at all angles, 

 and are even 'matted' together. In the latter cases the development 

 of antigorite seems to have made the rock as a whole more solid; 

 while in the former, as the fissility increases, ihe size of the flakes 

 diminishes." I infer, then, from examination of the specimens in 

 my collection, that, in the case of many, an augite-peridotite was 

 first more or less crushed ; next, the comminuted parts (most of the 

 olivine and some of the augite) were converted (still under some 

 pressure) into antigorite, which change often had the eff'ect of making 

 the rock less fissile than it had been. By the renewal of severe 

 pressure, the layers of residual augite and of antigorite might be 

 folded, or, if the former mineral had disappeared, the mass might be 

 rendered very fissile, with diminution of the size of the constituents, 

 as we find in the most ' slaty ' antigorite-serpen tines. In other 

 words, owing to the comparative uniformity of the rock, pressure 

 acts upon it as on any other nearly homogeneous mass, and produces 

 a very conspicuous cleavage. In the latter case, then, the stages 

 are : (1) augite-olivine rock, (2) the same somewhat cleaved, (3) anti- 

 gorite-rock, comparatively massive, and (4) antigorite-' slate ' (con- 

 sequence of renewed pressure). Thus many of the antigorite-rocks, 

 described in this paper, are, speaking in general terms, in the third 

 stage, while others are intermediate between it and the fourth, 

 which is completely reached by the most fissile specimens. 



But a peridotite, whether with augite or enstatite, or with both 

 these minerals, which has been converted into a serpentine like 

 that in the Ehaetian Alps, has in consequence become brittle ; hence, 

 under pressure, it forms a 'crush-breccia' with fragments of various 

 shapes, irregularly angular, as well as lenticular. These, if the 



^ I have examined specimens from New Zealand and from Norway. 



2 Gorner Grat, Fee Alp, Col de Vallante, etc. The same thing occurs 

 commonly in mica-s<-hists, wliich may be so crushed as to be distinguished, 

 with difficulty from ordinary phyllites. 



