Prof. Dr. T. G. Bonney — Troldolite, etc., in Aberdeenshire. 441 



of unaltered olivine still remain in fair number between the serpen- 

 tinous strings, which in several cases are shown to be remnants of 

 one crystal, by identity of colour when placed between crossing Nicols. 

 (2) Bastite, etc., viz. enstatite in various stages of alteration. This 

 mineral, as is common, includes some olivine grains like an in'egular 

 setting, and therefore must belong to a later epoch of consolidation. 

 We find it in various stages, from a characteristic enstatite, not very 

 much altered, to a kind of steatite speckled with earthy-looking 

 granules, and so rendered occasionally almost opaque. Sometimes 

 the steatitic product is isotropic, at others it appears as an aggregate 

 of very minute moderately bright-coloured granules. (3) Besides 

 these, the slide exhibits a fair number of grains, not veiy regular 

 in outline, either opaque or very dark brown in colour, most likely 

 representatives of the spinellid group, and in part probably 

 chromite.^ 



Thus the serpentine has been derived from an olivine-enstatite 

 rock, such as would be called a Saxonite by Dr. Wadsworth. 



The serpentine has been worked back to its junction with the 

 troktolite, which constitutes the main mass of the ridge. This is 

 usually a moderately coarse-grained mixture of purplish-grey or 

 whitish felspar and dull dark-green serpentine. Coarser varieties 

 occasionally occur; usually the felspar is to the serpentine in about 

 the proportion of three to two, but occasionally a ft-agment is seen in 

 which the former mineral distinctly predominates. A pyroxenic 

 constituent, if present, is very inconspicuous. The rock is boldly but 

 rather irregularly jointed and weathers with a brown rough 

 surface. 



I have examined a series of slides representing the average rock 

 of this part of the ridge, the coarser kind, and varieties respectively 

 rich in felspar and in serpentine. In the normal rock the felspar is 

 fairly well preserved, though rather cracked, probably by molecular 

 strains due to the alteration of the olivine constituent. It exhibits 

 the usual twinning of plagioclase, the groups of macles being com- 

 bined in both the Carlsbad and perikline types, and the large 

 extinction angles show that anorthite is present. The olivine has 

 been almost wholly converted into an olive-green to greenish-brown 

 serpentine with the usual structure ; only rarely does a fragment of 

 the original mineral remain unaltered. There is a certain amount 

 of diallage, more than one would have expected from a macroscopic 

 examination of the rock. This occurs in rather small irregular 

 "interstitial" looking grains, and now and then includes or is pierced 

 by a small crystal of the felspar, having been the last to consolidate. 

 There are also a few grains of iron oxide. One slide is traversed by 

 a vein of chrysotile. In the coarser varieties the felspar is not quite 

 as well preserved, but a fair amount of olivine is still unaltered, the 

 two minerals being more nearly in equal pi'oportions. In this there 

 is little or none of a pyroxenic constituent. 



1 Plate viii. figs. 2 and 3 (one of the peridotite of the Hartz, the other of that 

 from Christiania) in Dr. Wadsworth's Lithological Studies, part i. would very fairly 

 illustrate the serpentine described in this paper. 



