538 Prof. T. G. Bonney—The Effect of Pressure 



familiar to all students of rocks modified by pressure as one which 

 occurs when a mass, composed of grains of fairly uniform size and 

 strength, has been much compi'essed. The component grains seem 

 to have been flattened out and squeezed together. The other con- 

 stituent, the iron oxide, can be readily seen by the unaided eye, 

 forming black lines, up to about f inch in length, parallel with the 

 rock cleavage, like strokes made with a thick pen. These, on 

 microscopic examination, are found to be composed of grains or 

 granules, more or less aggregated in regular lines, and usually not 

 exhibiting a crystalline form, but being, so far as can be determined, 

 rather lenticular in outline. Generally they are black and perfectly 

 opaque. Sometimes, however, they appear slightly translucent and of 

 a brownish tint, and the adjoining matrix is stained with the same 

 colour, which penetrates for some little distance in a dendritic 

 fashion. There can, I think, be no doubt that these represent grains 

 of magnetite or possibly chromite, which have been crushed up and 

 arranged by pressure in their present form. Once or twice I note 

 lenticular clusters of larger and more definite flakes of the doubly 

 refracting mineral, but can find no distinct trace of a pyroxenic 

 constituent. 



At one spot we find in the low bosses which crop out from among 

 the scattered debris two rocks in association, both of which seem to 

 differ from the serpentine. The one is a dark dull-green chloritic 

 rock ; the other a talcose rock of a rather pale greyish colour. 

 The hardness of the former is about 2 ; the latter is still softer, being 

 easily scratched with the finger-nail. The 'chloritic' rock occurs 

 in a series of irregular reef-like masses, and I have no doubt that it 

 is intrusive in the talcose, though both evidently have been modified 

 by pressure and greatly altered from their original condition. 1 The 

 former, when examined under the microscope, is found to consist of 

 a flaky mineral, apparently belonging to the chlorite group, with 

 some flakes and irregular grains of an opaque iron-oxide, and (in 

 the junction-specimens) an occasional grain of a clear rather granular 

 mineral. The chloritic mineral has one well-defined cleavage 

 like a mica; it is moderately dichroic, showing a light dull-green, 

 with vibrations parallel with the cleavage-planes, and a very pale 

 straw colour with vibrations perpendicular to them. The dichroism 

 is more marked in a junction-specimen (where the flakes have a 

 more distinctly parallel arrangement), but this may only be due 

 to a difference in the thickness of the slides. The polarization tints 

 are low, but rather brighter in the latter specimen. Extinction 

 seems generally to take place parallel with the cleavage-planes, but 

 occasionally there is room for doubt on this point, and it appears to 

 be most complete at a very small angle with them. In the junction- 

 specimens the grains of iron-oxide are smaller, and more distinctly 

 linear in arrangement than in the other. In form they resemble 

 hematite. The third mineral has a rather granular structure, is 

 colourless, and gives low polarization tints. I have not seldom seen 



1 I am greatly indebted to Mr. J. Eccles, F.G.S., for verifying the opinion which 

 I had formed, and lor much additional information. 



