222 
Proceedings of Royal Society of Edinburgh. [suss. 
"while the potash magmas often appear to have possessed equal 
corrosive power over the whole of their surface in contact with the 
nock undergoing attack. A thin dyke, or a thin sill, of a basic rock, 
has made its way underground as a nearly parallel sheet, in some 
•cases over an area which may be hundreds of square miles in 
extent, and, what is still more remarkable, it has done so notwith- 
standing the fact that the rock invaded was at a lower temperature 
than the soda magma. Had the corrosive effect been equal over 
the entire surface in contact with the country rock, it must be obvious 
that the part first invaded, that is to say, the part nearest the conduit 
which gave emission to the fluid magma from below — would be the 
parts where the intruded rock would be very much thicker than at 
the points near the extremities. But many intrusive sheets appear 
to retain nearly the same thickness for a distance of many miles. 
The Whin Sill, for example, varies but little from the mean thick- 
ness throughout the greater part of the extensive area it occupies. 
The potash magmas, on the other hand, usually give rise to short and 
thick lenticular masses, and it is very rarely indeed that they appear 
as sheets with parallel boundaries. One is, of course, reminded by 
these facts of the similar behaviour of basic lavas, which may flow 
with comparatively little variation in thickness for thirty, forty, or 
even fifty, miles, while a lava stream of acid composition but rarely 
extends more than a very few miles from its point of emission, and 
in many cases does not get more than a few hundred yards away 
from that point before it comes to a standstill. Of course the 
temperature of the country rock must be an important factor in 
this connection in the case of all intrusive masses, even in those of 
trappean, as distinguished from plutonic, origin. Still, the fact 
remains, that potash magmas erode over their entire surface, so 
that they tend to eat their way outward in the form of gradually- 
enlarging wedges. It follows that the rock surfaces on either side 
•of one of these wedges may retain much similarity of form, and 
that the shapes of the opposite sides of a wedge may nearly or 
quite match, even though a considerable quantity of the interven- 
ing rock may have been removed. 
For the information of those who may wish to examine the 
evidence, it may be mentioned here that the best sections where 
the relations of the Ross of Mull granite to the country rock can 
