46 Correspondence—Prof. J. F. Blake. 
glomerate of Bull Bay is made of the underlying quartz rock. The 
fragments in the “agglomerates” or ‘“‘ conglomerates” of Llangefni 
are like some of the neighbouring rocks, the Bangor beds are full of 
fragments of rocks very similar to other parts of the same series, 
and the conglomerate of Moel Tryfaen is largely composed of the 
immediately preceding Cambrian slates. I do not, therefore, give 
much weight to any argument from such a grit. J only dealt with it 
because it was confessedly at first the only argument for there being 
“two groups of Precambrian rocks in Anglesey. Ifthe rock is dis- 
cussed, I have to say that of course Prof. Bonney’s description is 
accurate. The fragments do ‘‘much resemble some of the finer- 
grained schists of Anglesey ;” but they resemble those parts which 
have been entirely re-formed; such entire re-formation occurs some- 
times in bands parallel to the lamination, sometimes in veins crossing 
it, and is not confined to the older part of the series, but affects many 
other parts. The veins, like the rest of the rock, have formed under 
a pressure that has induced an orientation of the micaceous ingredient. 
We cannot, therefore, identify the fragments with any definite rock. 
4, The rocks near Llyn Irefwll. In this case, again, I should 
have said nothing except for the stress Dr. Callaway lays upon the 
locality. I twice failed to find the exact spot referred to, though 
the whole structure of the surrounding rocks seemed clear. Only 
by the minutest directions was I able to find a spot where some 
slaty-looking rock contained fragments of granite, and seems con- 
tinuous with the diabase which forms part of several bosses. Ail 
these figure as “slate” in Dr. Callaway’s paper. Several slides 
have been examined and prove to be diabase—hence “some of the 
slates of Dr. Callaway are diabase.” Where the granitic fragments 
are found, this diabase has become more or less hornblendic, or 
dioritic—or it may be that the containing rock is a distinct one. In 
any case, fragments of granite are contained in a slaty-looking rock 
which is not a slate. They must also be contained in a true slate, 
since such is their matrix as described by Prof. Bonney; but this 
slate is certainly not the diabase ridge figured as Pebidian by Dr. 
Callaway. It is really not worth while pursuing the question any 
further. 
5. With regard to the areas where there is a passage between Dr. 
Callaway’s lower and upper groups, his observations are rather 
special pleading, because ‘he selects two places, nearly the only two, 
where the succession is confessedly broken by faults, and gives these 
as examples ; whereas in both these places the supposed succession is 
stated to be entirely made out from what is observed abundantly 
elsewhere. 
6. As to the applicability of the Monian system to other regions, 
it is obvious that our first business is to thoroughly understand the 
development of a series of rocks in the place where the connection 
of one part with another is most fully displayed, and such a place is 
Anglesey, where the succession and stratigraphy in many places is 
comparatively clear. It is a later work to correlate other regions 
with the type, the probable result only being briefly indicated in my 
