﻿502 Rev. T. G. Bonney — P'dchstones and Felsites of Arran. 



spheroidal and columnar structures, and shows that while they are 

 due to the same cause, namely, contraction in cooling, they are 

 partially independent. 



The Birk Grlen Pitchstone shows a very remarkable sudden change 

 in the character of the rock. It is exposed in the bed of the burn, 

 and again at about twenty yards distance by the pathway. There 

 can be no doubt, though vegetation covers the intervening space, that 

 these are parts of one and the same bed. The pitchstone in the 

 burn is traversed by numerous joints, cutting it into rhomboidal 

 prisms, from about :| to |^ inch in diameter, one set of joints 

 occasionally predominating a little, so as to give a laminated struc- 

 ture to the rock. So marked and persistent is this jointing, that it 

 is almost impossible to obtain a specimen of any size. Under the 

 microscope the rock appears as a glass, either full of very minute 

 belonites, or exhibiting beautifully the batrachospermum-like groups;' 

 surrounded by clearer spaces, with very few spherulites, and no 

 banding of any kind. But the rock in the path has much fewer and 

 less regular joints, it shows a distinctly streaky structure, and a great 

 number of ill-defined spherulites, generally lying in the streaks. 

 Under the microscope it shows the same belonitic structure as the 

 other rock, except that this, as will be described below, is somewhat 

 modified by the conspicuous banded and spherulitic structure. As a 

 further proof of the identity of these two masses of rock, careful 

 examination shows that the pitchstone, where last seen by the path, 

 is becoming a little more regularly jointed, and in the bed of the 

 stream we find one or two nodes, as it were, in which the regular 

 jointing dies out, and a structure sets in, something like that seen in 

 the pathway. 



II. — Banded, Spherulitic, and Perlitic Structures. 

 In the paper mentioned above I endeavoured to show that perlitic 

 structure was only a variety of ordinary spheroidal structure, and as 

 this has been independently maintained by Mr. F. Eutley in a paper 

 read a few days afterwards before the Eoyal Microscopical Society,^ 

 and has since received further confirmation from Mr. S. Allport,^ I 

 shall take it as granted here. Banded or streaky structure is com- 

 monly supposed to be the result of tension * produced by flow in a 

 mass not perfectly homogeneous, and of this in many cases I have 

 no doubt. The evidence, however, which I am about to bring 

 forward will, I think, show that in certain cases it may result from 

 pressure. Spherulitic structure also is generally supposed to be the 

 result of crystalline forces. The evidence to be brought forward 



1 See Mr. S. Allport's description and plates, Geol. Mag. Dec. I. Vol. IX. p. 1. 

 See also p. 537. 



2 Monthly Microscopic Journal, 1876, p. 176. The author kindly refers to my 

 work so far as regards basalt only ; the reason being that I was obliged, in reading, 

 greatly to condense the paper, and dismissed perlite in one or two words, which escaped 

 his notice. 



^ Quart. Journ. Geol. Soc. yoI. xxxiii. p. 440. 



* Thus: the strain elongates oval masses and tends to draw them out into ropy 

 filaments. Furtlier, if a mass consisted of particles of various sizes, motion under 

 strain would have a tendency to pack like with like. 



