Prof. T. McKenny Hughes—Brecciated Beds at St. Davids. 307 
the group they are less frequent and their character and origin are 
more obscure. 
The beds in question sometimes appear to be made up of pebbles, 
but more often of pieces such as are commonly described as sub- 
angular, that is to say, pieces from which the corners have been 
removed while portions of the flat bounding surfaces remain, indicat- 
ing their original form. Sometimes they are all distinctly angular. 
When the fragments are derived from various rocks of different 
lithological character, or when fragments of one kind of rock only 
are imbedded in a matrix of a totally different character from the 
included fragments, it is obvious that the rock is composed of trans- 
ported fragmental material. 
But occasionally the constituent pieces are all of the same kind of 
rock, and even the matrix appears to. be composed of the same material. 
Tt is then often difficult to discover the origin of the mass, and to 
make out whether we have a rock made up of worn and transported 
fragments or only a mass brecciated in place. 
There are, however, some tests which may be applied with con- 
fidence even in these most difficult cases. 
In mapping any district where exposures of rock are few and far 
between, it is often very important to determine whether a large mass 
of rock is in place or only the projecting portion of a boulder or 
tumbler. Now if the rock be cleaved, it is often sufficient to observe 
‘the direction of the cleavage planes; for, if the cleavage, which as 
a rule is much more constant than bedding, does not coincide with 
the general direction of the cleavage in the surrounding district, we 
may fairly suspect that the mass is not in place. 
So in the case where a conglomeratic or brecciated mass is com- 
posed of pieces of rock of one kind only imbedded in a matrix of 
the same; if the fragments are derived from a cleaved or foliated 
or schistose rock, then, as the pieces in the conglomerate or breccia 
are irregularly thrown together, the divisional planes are seen in it 
at all angles to one another and not approximately parallel, as in the 
parent rock. 
It is necessary to observe this point in drawing, for instance, the 
lower boundary of the Carboniferous beds in the Alais Coal-field in 
the South of France, where the basement bed is made up entirely of 
fragments of the underlying schist compacted into a tough mass, 
which it would be difficult to distinguish from the underlying solid 
rock, except by the application of the test above described. 
There still remain the difficult cases of certain breccias and 
brecciated conglomerates in homogeneous rocks. It is well known 
that we have numerous cases of brecciation-in-place in chalk, 
especially in the Chalk Rock. In chalk the continuity of a layer of 
tuberous flints will sometimes tell us that we have brecciation in 
place, while in an adjoining section a few scattered ferruginous flints 
in the re-compacted chalk tell of old gravel beds from which they 
were derived, and prove that the chalk also must have suffered 
erosion and transport. 
In the older formations also we have examples of the breaking up 
and recementing of rock masses in place. In some the infiltering water 
