OLDEK PERTDOTITES OF SCOTLAND. 405 



is a dark-green serpentine. Examined with high powers of the 

 microscope, this serpentine is seen to be filled with black and brown 

 enclosures, some rod-like and stellate in form, others of a tabular 

 character. Sometimes these inclusions are arranged in one set of 

 parallel planes only ; in other cases they lie in two sets of planes 

 intersecting one another. There can be no doubt that much of this 

 serpentine is pseudomorphous after olivine ; but some of it may 

 replace enstatite. 



The micaceous mineral presents very different conditions in 

 different portions of the rock. Most frequently it appears to have 

 been changed into a creamy yellow, amorphous substance, which by 

 polarized light shows all the characters of an alteration-product ; 

 but in the fresher examples of the rock it exhibits the peculiar 

 outlines and the strong basal cleavage of a mica. From ordinary 

 biotite, however, it is distinguished at once by its pale colour, a 

 faint buff-yellow, and its feeble pleochroism, which is, however, 

 sufficiently well marked to be unmistakable. In places the fresher 

 part of this mineral is rendered black and almost opaque by the 

 abundance of tabular inclusions which it contains ; these appear to 

 be arranged in planes parallel with the basal plane, that is to say 

 in the direction of the principal cleavage. The distribution of these 

 inclusions is strikingly local, some parts of the mica crystals being 

 almost entirely free from them, while in adjoining portions they 

 have become so frequent as to entirely destroy the translucency of 

 the crystal. 



Small grains of magnetite are found scattered through all the 

 minerals of the rock ; but of accessory minerals there are only a few 

 traces. Yery rarely indeed could any minerals be detected which 

 might be regarded as ^""teration-products of felspar. 



By drawing, with the aid of a camera-lucida, the outlines of the 

 crystals in a section of the rock, and cutting out and weighing the 

 fragments of paper representing each mineral, an estimate was 

 formed of the proportions which the several minerals bore to one 

 another in the rock. The operation, repeated in the case of a 

 number of sections, so as to afford a good general average, gave the 

 following percentages : — 



Hornblende 58*5 



Serpentine 22*0 



Altered mica 18'5 



Magnetite and accessory 1 -.^r. 



minerals J 



This result appears at first sight very different from what might 

 be expected from a macroscopic inspection of the rock, which would 

 lead one to regard the conspicuous mica as the predominant con- 

 stituent. But minerals with a strong cleavage like mica are very 

 apt to make a much greater show on fractured rock-surfaces than 

 their actual abundance in the rock entitles them to do. 



The microscopic structure of the rock is a very marked one. The 

 hornblende-crystals enclose rounded grains of serpentine and crystals 



Q.J.G.S. No. 163. 2f 



