﻿692 PKOF. T. G. BONNET AND MISS C. RAISIN ON THE [NoV. I905, 



original crack, thus marking its position, or as an imperfect coating 

 on the strings, and that the process of alteration, after these have 

 formed, seems to be checked for a time, since we find the meshes so 

 often occupied with quite normal olivine. When this changes, the 

 distribution of the iron-oxide is less regular, though it commonly 

 forms a more or less central aggregate of granules ; the serpentine 

 is more fibrous and more variably arranged, being sometimes a 

 little larger than that in the strings, sometimes so very minute 

 that the mass between crossed nicols either behaves like a colloid, 

 or exhibits the feeblest glimmer of specks, like the sky in parts 

 of the Milky Way. An occasional result of the second change is to 

 disturb the regularity of the network, and practically obliterate the 

 structure of the olivine. The amount of iron, rendered visible by 

 these changes, varies considerably, and no doubt depends on the 

 percentage present in the original mineral; but it is sometimes 

 more abundant in the strings than in the meshes, sometimes the 

 reverse, and occasionally appears to remain in an ultra-microscopic 

 form, giving a pale-yellow or green tinge to the one, the other, or 

 both. In one specimen almost all the iron may be deposited in the 

 original cracks; in another it may form a rather broad reddish-brown 

 border to the interstitial serpentine. Colourless serpentine more 

 commonly gives as polarization-tints whites or bluish-whites of the 

 first order, the pale greenish-yellow variety producing generally 

 the yellow or orange-reds of the same. This, we think, is not 

 merely a result of size and thickness, for the two may occur in the 

 same slice, 1 but is, we believe, due to the colouring-matter, which 

 often does not produce any perceptible pleochroism. 2 Extinction is 

 generally straight, but in both cases, and perhaps more often 

 in the latter variety, it makes angles, up to about 15°, with the 

 edge of the fibre or flake. 3 We have failed to note any other 

 characteristic distinction of these varieties, if such they be. 



In the diamantiferous breccia of South Africa 4 the olivine usually 

 occurs in single grains, more or less converted into serpentine. In 

 thin slices it is either colourless, or yellowish, or a pale green 

 (the tint depending to some extent on the thickness of the slice) ; 

 it rarely throws out granules of iron-oxide, and in changing to 

 serpentine exhibits some variations. These have been described 

 by the late Prof. Carvill Lewis, 5 whose statements one of us can 

 corroborate, except that he has neither met with a good cleavage 



1 Of this I have more than one instance ; one of the best comes from the 

 compacter form of a rather streaky serpentine at Lankidden Cove, the Lizard. 

 [T.G.B.] 



2 Some specimens from Anglesey in Miss Raisin's collection, in which the 

 serpentine is a pale buff-yellow by ordinary light, show a pleochroism from 

 that tint to pale green. 



3 Dana, « System of Mineralogy ' 6th ed. (1892-1900) p. 669, gives serpentine 

 as a monoclinic mineral. 



4 My cabinet contains twenty-seven slices from Kimberley (some from over 

 1300 feet deep) and Newlands. As I have elsewhere pointed out, this rock, the 

 so-called kimberlite, cannot be, as was once supposed, an altered peridotite. 

 [T. G. B.] 



5 ' The Genesis & Matrix of the Diamond J 1897, pp. 14-16. 



