Prof. T. G. Bonney— Welsh Pre-Cambrian Rooks. 299 
reasons why I so did, and intend so to do on any future occasion that 
may arise. Evidence in the field, doubtless, is sometimes so clear 
that we need not call in the microscope. No one, however, can assert 
this to be the character of the rock testimony in the district for some 
miles on either side of the Menai Straits. Professor Hughes’s pub- 
lished sections look very clear; but I may remark, without offence, 
that they are more distinct on paper than in nature. Further, the 
questions debated in these papers are exactly those where microscopic 
study is essential in order to attain to anything like asure conclusion. 
I am well aware that mountains must not be looked at through the 
microscope only—I never have so worked or will so work at any 
petrological question ; but I do assert, after seven years’ experience 
with the instrument, that all conclusions as to the minute constituents, 
amount of metamorphism, and precise nature of these old rocks, must 
be thoroughly tested with the microscope before we can accept them. 
I know from this experience that even well-trained eyes—those I 
mean of observers versed in the special study of rocks, both with 
and without the microscope—have been occasionally deceived, when 
they alone were trusted. Professor Hughes seems to reason 
(for otherwise his complaint is meaningless) as if the results of 
microscopic lithology were at present all in uncertainty. Were this 
the case, years of hard work, by not unqualified observers, would 
have been lamentably wasted. But though, as all such observers know 
well, we have yet much to learn, there are many points on which 
we are certain of our conclusions. What would have been thought, 
some sixty years since, of any geologist who had insisted on corre- 
lating strata “by tracing the great rock-masses in the field” only, 
without paying any attention to their fossil contents? Yet micro- 
scopic lithology is now as important a study, and in about the same 
position as Paleontology was then. 
I may, indeed, even venture to say that the internal evidence 
of Prof. Hughes’s paper leads me to conclude that he is so sceptical 
of the value of microscopic study as to be quite insensible to consi- 
derations which would seriously disquiet most petrologists. ‘T’o take 
one example only: we have in N.W. Carnarvonshire, below the 
Cambrian conglomerate, (1) a granitoid series, (2) a mass of quartz 
felsites representing old rhyolitic lavas, (8) a group of slates and 
breccias. Dr. Hicks regards these as three totally distinct series 
separated by very considerable breaks. I do not feel quite so certain 
of the great separation of (2) and (3); but Prof. Hughes considers, 
as he says, that “there is no unconformity visible between the groups, 
whether we accepted the brackets drawn by Dr. Hicks, or those 
which I proposed.” ! Further cn, speaking of the granitoid rock of 
Twt Hill, in the lowest part of the series, he says, “ It is doubtful 
whether any specimen taken from the heart of this rock has 
exhibited clear evidence of a fragmental origin.”” I suppose, then, 
this means that Prof. Hughes is not satisfied whether the rock is 
a granite or a granitoid gneiss. Apparently, however, he does not 
perceive that in expressing this doubt, he is placed in the following 
1 Quart. Journ. Geol. Soc, vol. xxxiv. p. 682. 
