﻿Rev. T. O. Bonne y — Pitchstones and Felsites of Arran. 501 



Pitclistones, Tormore sliore. — The soutliernmost of these dykes 

 exhibits banded, laminated and rudely columnar structure. The 

 first is generally parallel to the upper and lower surface ; so also is 

 the second, but not invariably, as the two structures do not always 

 correspond in direction, and occasionally cut at an angle of 

 nearly 80°. 



Fig. 1. — Diagi-am of intersection of streaky and laminated structure, from Lower 

 Pitchstone, Dunfion. 



A. Laminated structure, rather curved. B. Streaky structure. 



A dyke of brown pitchstone to the north of the last, about four 

 feet wide, shows banded structure very clearly on the southern side, 

 where there is an intruded basalt dyke. This structure is barely, if 

 at all visible on the northern. 



The above cases seem to show that, though the fissile and 

 banded structures are often coincident by reason of a common re- 

 lation (which will be presently discussed) to the surface of the mass, 

 yet they are of independent origin ; and that the former is really 

 only one special case of a series of phenomena due to contraction 

 of the rock in cooling. Many other instances showing the relation 

 of fissile structure to tabular and columnar jointing, and that 

 between these and spheroidal structure, might readily be adduced 

 from the various Arran rocks ; but it is perhaps needless to multiply 

 them. One curious case, however, in a pitchstone from Moneadh- 

 mhor Glen, may be worth a brief description, as well as a remarkable 

 change in structure in the Birk Glen Pitchstone. The first of these 

 pitchstones (exposed in two places in the stream, both apparently 

 parts of one irregular bossy intrusion), is very closely and irregu- 

 larly jointed, generally platy towards the exterior and passing into 

 a columnar structure in the interior. A hand-specimen from the 

 latter part illustrates the connexion between jointed and spheroidal 

 structures. It exhibits three or four fairly well defined columns, 

 while in other parts the joints have no definite direction. The 

 largest column, about f inch in diameter, exhibits an external platy 

 structure ; within this is seen at one end a very shallow cup, 

 elliptical in outline. The other end shows an irregular dome-like 

 ball. On following the surface of this downwards, it is seen, 

 where becoming lost to view, to be again curving inwards, so that 

 it resembles a rather distorted egg, partly sunk in the column. 

 Somewhat similar structures are exhibited by the ends of the other 

 columns. I conceive that this specimen supports my view of 



