308 Prof. T. WcKenny Hughes—Brecciated Beds at St. Davids. 
hardens the rock along the joints so that during subsequent weather- 
ing the unaltered rock perishes more rapidly than that part which 
has been affected by infiltration, and projecting ridges mark the 
lines of joint, as good mortar and the contiguous lime-strengthened 
part of the stone often stands out when the central portions of inferior 
building stones have crumbled away. In other cases the rock perishes 
most along the joints, so that the solid pieces between the divisional - 
planes are more and more reduced in size, the corners are rounded 
off and at last they appear as subangular or rounded nuclei in a rotten 
matrix of somewhat similar material. Not exactly similar—for the 
process of decomposition does not often leave the rock as it was. 
A removal of some of the alkalines along the joints could probably | 
in the case of granite be detected, and the chemist and mineralogist 
would report that the included fragments were imbedded in a matrix 
of different composition. 
Even in this case it may be possible to determine whether the rock 
is made up of transported material or only brecciated in place. For 
it is extremely improbable that under any combination of circum- 
stances fragments could be arranged by nature in such a manner that 
the flat bounding surfaces of consecutive pieces should over and over 
again touch the same plane, not being the plane of bedding; and if a 
sufficient surface is exposed to enable us to make out that the planes, 
which so limit the apparently included fragments, coincide with the 
joints which affect the whole rock, then the conclusion is inevitable 
that we have in such a case only the chemical destruction of the 
rock along the joints which have allowed a freer’ percolation of the 
surface water. a 
Brecciated Granitoid Rock, near Brynygarn, St. Davids. 
Now there is a face of granitoid rock exposed on the east. side of 
the Alan valley, 8.8.W. of Brynygarn near St. Davids, which forms 
so marked a feature in that mass, that it has been taken as an horizon 
useful for reference when describing the exact position in that section 
where the Dimetian ends; though of course, if it be not a true 
breccia, it has no more value than any other mere geographical 
feature. The apparently included masses are of various sizes from 
