148 G. J. Williams—Parys Mountain, Anglesey. 
With regard to the cause of this particular alteration of the chalk, 
I cannot say much as I have not analysed specimens of the unaltered 
chalk. According to Hume’s analyses,’ the white limestone (Upper 
Chall) that he analysed did not contain more than one per cent. of 
impurity, and this would not be enough to supply the silica in the 
metamorphosed rock. That the dyke suppled the heat necessary, that 
is, that the metamorphism took place subsequently to the formation 
of the ‘neck,’ I feel convinced, as the conversion of chalk into 
crystalline limestone is chiefly found in connection with dykes later 
than the chief outflow of lava over the county, and I can only 
suggest that there has been a mixture of the dyke with the chalk in 
different proportions. This would explain the varying mineral 
composition of the different specimens, the iron of the ferro-magnesian 
minerals being thrown out in the form of magnetite, the magnesium 
going to form the diopside. The parts where there was an abundance 
of magnetite would be those with the largest amount of mixture with 
the dyke. 
In conclusion, I should like to thank Miss M. K. Andrews for the 
loan of sections and specimens, Drs. Cullis and Flett for advice and 
suggestions, and Mr. Waldron Griffiths, of Cirencester, for the micro- 
photographs illustrating this paper. 
EXPLANATION OF PLATE VY. 
Fic. 1.—Diabase: Scawt Hill, co. Antrim. x 15. The zoning of the augites is 
well seen on the right in the two long crystals. The lighter part seen in the 
section is felspar; iron-ores are black and opaque, while apatite is seen in 
six-sided section. 
Fie. 2.—Cale-silicate Hornstone: Scawt Hill. x 15. This shows the difference 
due to the varying amount of magnetite present and the sharp line marking off 
one part from the other. 
Fic. 3.—Cale-silicate Hornstone: Scawt Hill. x 25. The idiomorphic character 
of some of the minerals is well seen near the centre of the section, where 
wollastonite, zoisite, and scapolite are enclosed by the clear calcite. 
Il.— Nort on tHe Grotogican AGE oF THE SHALES OF THE Parys 
Mountain, ANGLESEY. 
By G. J. Witurams, M.I.M.E., Assoc. Inst. M.M., F.G.S. 
(¥VHE rocks of the Parys Mountain, which lies about two miles to 
| the south of the town of Amlwch in Anglesey, were described in 
1878 by the late Mr. T. Fanning Evans, H.M. Inspector of Mines, 
who regarded the Parys Mountain ‘‘as an outlying piece of ground, 
geologically identical with the region in which the great slate-mines 
of Festiniog occur, and from which it differs only in being developed 
on a much smaller scale, and in being highly metalliterous.” ? 
The Festiniog slate beds are well known to be of Llandeilo age, 
but at the time Mr. Evans wrote, thirty years ago, little was known 
of the black shales of North Wales or their fossil contents. 
In the second edition of Ramsay’s ‘“‘ Geology of North Wales,” 
p- 248, reference is made to a band of very black shale containing 
1 Q.J.G.S., vol. li (1897). 
2 Trans. Manchester Geol. Soc., vol. xiv (1878), p. 360. 
