A. Harker—Geology of Mynydd Mawr. 221 
“eyes”? does not appear to be due to the original growth of the 
coral, for the sides of the nodules are seen to consist of sections of 
the coral. This may be owing to chemical solution of some parts 
of the limestone, but appeared to me to be caused by the actual 
shearing off of a portion of the coral. This supposition requires 
confirmation, and it is probable that a fuller examination of the 
district will yield the requisite evidence. 
The structures seen in these North Devon rocks remind one of 
those described by Dr. Bonney as occurring at Tor Cross in 8. Devon 
(Q.J.G.S. vol. xl. p. 1). He brings forward proof to show that in 
that region “there is no valid evidence of a passage from schist to 
slate.” The occurrence of rocks in North Devon having all the 
. mechanical structures of true schists, without possessing their 
peculiar chemical composition, bears out this conclusion. In con- 
nection with this, it is of interest to notice that whereas in South 
Devon, where phyllites are found associated with normal schists, the 
sedimentary rocks are largely penetrated by igneous intrusions, this 
is not the case in North Devon, where such intrusions are very rare. 
One mass of quartz felsite, which has been described by Dr. Bonney 
(Grou. Mac. 1878, p. 207), does occur at Bittadon, and he states 
that it is ‘‘ affected slightly by cleavage.” It was therefore intruded 
prior to the last earth-movements of this area. In some parts of the 
rock there is a parallelism of the alteration products which have 
been developed along the line of cleavage, but unfortunately the 
portion of the rock which has undergone the greatest amount of 
cleavage is so decomposed that no specimen could be obtained 
sufficiently firm for slicing. The apparent absence of schistosity 
in this rock can be however accounted for on the supposition that at 
the place where it is exposed, the mass forms an “eye” which has 
not undergone any great change. It would be of interest to know 
if this rock is exposed elsewhere, and if so, under what conditions 
it is there found. 
I am indebted to Mr. E. J. Garwood, B.A., F.G.S., for the use of 
photographs displaying many of the structures which I have 
described above. 
IVY.—Nores on tHE GEoLoGY or Mynypp Mawr AND THE 
NANTLLE VALLEY. 
By Aurrep Harker, M.A., F.G.S., 
Fellow of St. John’s College, Cambridge. 
YNYDD MAWR, about three miles west of Snowdon, is an 
abrupt rounded hill, 2300 feet high, separating the valleys 
of Nantlle and Cwellyn. A reference to the maps of the Geo- 
logical Survey (75 N.E. and N.W.) shows it to be due to an 
isolated boss of “intrusive hornblende-porphyry ” in the form of 
a rounded parallelogram, a mile and a half in its longest diagonal. 
Dr. Hicks* has mapped this patch as Pre-Cambrian, and included 
it in his Arvonian system; but apart from its position, breaking 
1 Q.J.G.S. vol. xxxv. p. 297, 1879. 
