562 Dr. O. Callaway—On Blake's “ Monian System.” 
system in Anglesey? In arguing for two groups, I assigned some 
importance to the evidence of included fragments. Prof. Bonney 
(supra, p. 235) states that the Llanfechell Grit (Newer Archean) 
contains “detrital materials almost certainly derived from the older 
gneissic and schist rocks of this part of North Wales.” Again, in 
the Quarterly Journal for 1884, p. 576, I contended that the basement 
Paleozoics near Tywyn contained fragments of slaty rock, and that 
a mass of the same slate in situ included rounded pieces of the 
adjacent granite. Prof. Bonney, after microscopic examination, 
unhesitatingly confirmed my identifications. Prof. Blake, however, 
contests both points. He says that none of the fragments in the 
Llanfechell Grit are “indubitably schists.” Prof. Bonney, at my 
desire, has re-examined the rock, and thus writes :— 
“My description is accurate. There are several fragments of a 
very fine-grained schist consisting of quartz and a flaky green 
mineral, which is either chlorite or an altered biotite, so far as I can 
make it out. These much resemble some of the finer-grained 
schists of Anglesey in my collection. Of course, in some crystalline 
schists one cannot say how much of the structure may be due to 
mineral segregation, or to veining subsequently modified; but in 
the ordinary sense of the words these are bits, not of vein sub- 
stance, but of schist. Probably Prof. Blake was unlucky with his 
specimen.” In reference to the last suggestion, I may say that both 
professors have studied the same slide. 
The slate near Tywyn is regarded by Prof. Blake as a “ diabase.”’ 
The slides of this slate originally described by Prof. Bonney have 
been again examined by him. This is what he says about them :— 
‘My descriptions are accurate. There must be some misunder- 
standing about the locality, for I cannot realize the possibility of 
such a mistake being made as to call this rock a diabase.” Prof. 
Blake? appears to have no doubt that his “diabase” is the same as 
my “slate,” and from his description I should think he was right ; 
though his examination of the ground was so incomplete that he 
did not even find the conglomerate and grit? which rest on the 
eastern side of the granite axis. If then Prof. Blake has rightly 
identified his “diabase” with my “slate,” we are driven to make 
a choice, on the microscopic question, between him and Prof. 
Bonney. Prof. Blake will forgive me if I prefer Prof. Bonney’s 
opinion to his. 
Having thus disposed of the argument from included fragments, 
Prof. Blake attempts to prove that there is a passage between my 
lower and upper groups. It would take a paper of many pages to 
discuss this point. Whatever there may be in spots I have not 
visited, I am bound to say that in those districts known to me Prof. 
Blake’s efforts to connect the gneissic rocks with the slaty appear 
1 In a note to p. 488, Prof. Blake has ‘‘ little doubt’ that I sent to Prof. Bonney 
fragments included in the Paleozoic in mistake for specimens of the rock in situ ! 
I make no comment on this suggestion, as I am unwilling to believe it was seriously 
intended. 
* Several specimens were described in my paper (Nos, 124, 125, 126). 
