502 Ree. T. G. Bonney — Pitchstones 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 Gien Pitchstone shows a verv 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 
prisrns, from about A to f 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, 1 
surrounded by clearer spaces, with very few spherulites, and no 
banding of any kind. But the rock in the path lias much fewer and 
less regulär 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 splierulitic structure. As a 
further proof of the identity of these two masses of rock, careful 
examination shovys that the pitchstone, where last seen by the path, 
is becoming a little more regularly jointed, and in the bed of the 
strearn we find one or two nodes, as it were, in wliich the regulär 
jointing dies out, and a structure sets in, something like that seen in 
the pathway. 
II. — Banded, Splierulitic, and Perlitic Structures. 
In the paper mentioned above I endeavoured to sbow that perlitic 
structure was only a variety of ordinary spheroidal structure, aud as 
this has been independently maintained by Mr. F. Butley in a paper 
read a few days afterwards before the Royal Microscopical Society, 2 
and has since received further eonfirmation from Mr. S. Allport, 3 I 
shall take it as granted here. Banded or streaky structure is com- 
monly supposed to be the result of tension 4 produced by flow in a 
mass not perfeetly homogeneous, and of this in many cases I have 
no doubt. The evidence, however, wliich I am ahout to bring 
forward will, I think, sliow that in certain cases it may result from 
pressure. Splierulitic 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 heilig that 1 was obliged, in reading, 
greatly to condense the paper, and dismissed perlite in one or two words, which escaped 
bis notice. 
3 Quart. Journ. Geol. Soc. vol. xxxiii. p. 440. 
4 Thus : the strain elongates oval masses and tends to draw them out into ropy 
filaments. Further, if a mass consisted of particles of various sizes, motiou under 
strain would have a tendeucy to pack like with like. 
