﻿696 PROF. T. G. BONNEV AND MISS C. RAISIN ON THE [Nov. I905, 



as it is in altered saxonites and Iherzolites from Ayrshire and 

 Portsoy, from the Alps and the Apennines. 1 It occurs in Anglesey 

 (exceptionally), in Aberdeenshire, and in the well-known rock of 

 Baste in the Harz. In these cases, the enstatite seems to be rather 

 readily changed into a whitish steatite, with a muddy brownish 

 aspect under the microscope. The included olivine is usually a 

 nou-ferriferous variety. 2 



Y. Monoclinic Pyroxenes. 



These, when they are associated with the orthorhombic species, 

 are much less readily converted into serpentine, and, as in it, the 

 change is practically restricted to the varieties poor in iron (diopside 

 and sahlite), apparently not occurring in the aluminous varieties 

 (for example, ordinary augite). Specimens in our collections from 

 Coverack in Cornwall, from one or two localities in the Alps, the 

 Apennines, and the Yosges, retain the augite practically unaltered 

 but the change is very well illustrated in the noted 'Eozoonal' rocks 

 of Canada, which are mainly composed of calcite or dolomite, and 

 malacolite or sahlite passing into serpentine. 3 A description of one 

 specimen (from a wood by a road approaching Cote St. Pierre) will 

 serve to illustrate the general character of the change. Here 

 nodules of a whitish augite, more or less converted into serpentine 

 (light green), are scattered in a marble containing irregularly- 

 distributed grains of augite or serpentine, which sometimes occur in 

 fairly -marked bands. The augite in a slice is present in more or less 

 rounded grains, which form a continuous band at one end. It 

 exhibits sometimes the characteristic, nearly-rectangular cleavage 

 of an augite, but more often a single cleavage, not so close as in a 

 diallage. At the edge of the band and in many detached grains it 

 passes rapidly into serpentine. This occasionally forms along cracks 

 (not always cleavage-planes) and its fibres are sometimes twisted as 

 in a cord, but more frequently are growing at right angles to a 

 surface or to an internal crack. These afford polarization-tints up 

 to the yellow of the first order, but the inner part of the grains is 

 occupied by a filmy mineral with very low tints, sometimes all but 

 black between crossed nicols, which now and then is pierced by a 



1 These remarks, it must be understood, apply only to what I have myself 

 seen and collected. [T. G. B.] 



2 The fusion-point in olivine is higher than in enstatite, but the different 

 amounts of iron and the presence of water may lead to variations. 



3 Remarks founded on ten microscopic slices (coll. T. G. B.) ; see Geol. Mag. 

 1895, p. 292. As Mr. Coomaraswamy found forsterite with sahlite in the 

 Tiree Marble (Quart. Journ. Geol. Soc. vol. lix, 1903, p. 91), and the two 

 minerals (apart from cleavage) have a close resemblance, I wish to say that the 

 possible occurrence of an olivine in these Canadian rocks has been present to 

 my mind for several years (Geol. Mag. 1895, p. 297) and has not been forgotten 

 on the present occasion. It is impossible to prove that forsterite may not have 

 occurred or be represented by some residual granules ; but I can say that, 

 whenever these afford an edge from which extinction may be measured, this is 

 oblique, and that bands of pyroxene with characteristic cleavage do pass rapidly 

 into serpentine. [T. G. B.j 



