194 H. HICKS ON CAMBRIAN CONGLOMERATES IN 
of the granitoid rocks from below. These have been submitted to 
Prof. Bonney for examination with the microscope, see his Notes 
1-6, pp. 201-203, and his conclusions agree entirely with mine, 
that the pebbles are identical with the rocks below, that the pecu- 
liarities exhibited by the granitoid rocks of the ridge are shown 
equally clearly in the pebbles, and that it is evident the granitoid 
rocks were much in the condition in which they are now found before 
the fragments now lying in the conglomerate were broken off from 
the original mass *. Even superficially these pebbles show evidences 
of this fact. The crushed appearance exhibited by the parent rock 
is equally clear in the pebble; and that infiltration of a green 
material along the joints, so commonly found in the Dimetian, had 
taken place in Pre-Cambrian time, is made certain from finding that 
it is equally abundant in the fragments. 
Subsequent crushing is occasionally indicated by fracture-lines 
across the pebbles, usually filled by quartz; but these lines are quite 
independent of the indications of the original crushing, which seems 
to have affected the whole structure of the rocks even to a point 
only revealed under the microscope. It has been frequently stated 
that the quartz in the Dimetian has a dirty appearance, and that this 
is due to an immense number of inclusions, chiefly of vesicles arranged 
in well-marked lines. These vesicles are supposed to indicate that 
the quartz was formed in the presence of abundant vapours. This 
view of their origin is now so universally accepted that I fear it 
will be considered rank heresy even to suggest an additional cause. 
Still, after a prolonged study of a large series of slides made from the 
Dimetian rocks and also from the older gneisses of Scotland, in which 
the vesicles occur in equal profusion, I must confess that the above 
view does not appear to me to be sufficiently satisfactory to account 
for their unusual abundance, or for the condition generally of the 
quartz in the old rocks. We know that these rocks have been 
crushed to a microscopical degree, and that this has allowed infiltra- 
tion to take place to such an extent that the majority of the minerals 
have undergone a process of decomposition. One result of these 
decompositions would be that liquids with powerful solvent properties 
would be produced, which by a kind of capillary action would 
pass along the minutest fissures and so attack the quartz and cause 
decay. The quartz shows abundant evidence of having been broken 
up, and the fragments are frequently separated by the deposition of 
secondary minerals. The lines of fracture are also traceable from 
one fragment to another by thin strings of these secondary minerals ; 
and it is an important fact also that occasionally these strings are 
seen to connect lines of vesicles in different fragments. The 
* As it is a matter of great importance that no possible doubt should exist as 
to the identity of the pebbles in the conglomerate with the rocks below, Prof. 
Bonney took the precaution to examine all the slides submitted to him, distin- 
guished by numbers only, before looking at the rock-specimens, and it is 
undoubtedly a highly satisfactory proof of the great value of the microscope 
in these inquiries that the results required no modification in any of the main 
facts when it was found from what areas the specimens had been obtained. 
In several of the slides also there was no evidence to guide him as to whether 
they had been cut actually from the parent rock or from a pebble. 
