Rev. T. G. Bonnei/ — Pitchstones ancl Felsites of Arran. 501 
Pitchstones, Tormore shore . — The southernmost 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°. 
A. 
Fig. 1. — Diagram of intersection of streaky and laminated structure, from Lower 
Pitchstone, Duufion. 
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, 
wliere 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 Gien, may be worth a brief description, as well as a remarkable 
change in structure in the Birk Gien Pitchstone. The first of these 
pitchstones (exposed in two places in the stream, both apparently 
parts of one irregulär bossy intrusion), is very closely and irregu- 
larlyjointed, 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 irregulär 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 
