300 Prof. T. G. Bonney— Welsh Pre-Cambrian Rocks. 
dilemma. If the rock is granite (and that carries some of the rock 
at Port Dinorwic), how do we explain the immediate succession to it 
of a rock which is almost a rhyolite; for surely the one would 
naturally be regarded as connected with the other? If, however, 
the rock is a gneiss, and simply the lowest member of a continuous 
series, how do we explain the presence above it of a scarcely altered 
rhyolite, and above that of a very slightly altered series of slaty and 
other beds? These are serious difficulties which I think should not 
have been passed over in silence. It is also rather new to me to 
find apparently a brecciated aspect in a lava flow used as of classi- 
factory value (p. 687); indeed the author’s petrology in this page 
puzzles me more than once. 
I proceed then to deal in detail with some of his criticisms, com- 
mencing with the granitoid series which extends from T'wt Hill to 
above Port Dinorwic. There are certain grits and conglomerates 
exposed on Twt Hill (to N.E. of the summit), at intervals over the 
tract of granitoid rock towards the N.E. of Carnarvon, and lastly 
near Careg Goch, above Port Dinorwic: these he holds to be 
one and the same set of beds, and considers them representatives 
of the Cambrian conglomerate, brought into their present position 
by faulting. As to the identity of these beds, I feel now satisfied,’ 
but I am by no means able to admit that they can be reckoned 
with the Cambrian series. At Twt Hill we have in more than 
one place a fairly continuous section from the true granitoid 
rock to the conglomerate. Obviously the possibility that the 
conglomerate might be made up from the granitoid rock, like the 
Arkose of Auvergne, at once presented itself; but though I have 
twice examined the section, the second time most minutely, I can 
find no break between the two rocks. Dr. Callaway, after examining 
the section ‘‘inch by inch,” confirms my view, stating also that he 
has found a like conglomerate in a similar position in Anglesey (p. 
118). The occasional appearance of fragments of schist among the 
quartz pebbles, which chiefly compose the conglomerate, doubtless 
seems to favour the view that it is later than the granitoid rock ; 
still, this evidence is not conclusive, for we do not know the relation 
of that to the schist. ven if it were proved that this conglomerate 
were made up from the materials of the rock on which it rests, I 
could not believe it to be of Cambrian age, because the microscope 
shows that its matrix, and the finer bands intercalated with it, are 
much more highly altered than is the case with the Cambrian 
rocks. Further, I have never yet seen a felsite pebble in this con- 
glomerate or detected a fragment under the microscope, yet that 
rock abounds in every known exposure of the Cambrian conglome- 
rate; and at Twt Hill itself, within a short distance, there is some 
* I was formerly inclined, from the evidence of strike and in defect of proof of 
faults, to place the Careg Goch grits on a lower position than those of Twt Hill, but 
subsequent examination has shown me that I was misled (partly by defects on the 
map) on the former occasion, and that there is good reason to infer the presence of 
faults, which cause the general trend of the beds to correspond more nearly with the 
line of the ridges, 
