CHAP. X 
sr. jDav/D’s 
153 
had lain in tliat direction. There are donbtless intrusive as well as 
contemporaneously interhedded masses in the rough ground between Pen- 
niaen-inelyn and Treginnis. To separate these out would be a most 
interesting and beautiful piece of mapping for any competent geologist in 
possession of a good map on a sufficiently large scale. 
The interhedded lavas, so far as I have had an opportunity of studying 
them, appear to present remarkable uniformity of petrographical characters. 
Megascopically they are dull, fine-grained to compact, sparingly porphyritic, 
ranging in colour from an epidote-green to dull blackish-green and dark 
chocolate-brown. Some of them are finely porphyritic from the presence 
of small glistening surfaces which present the colour and metallic lustre of 
haematite and yield its characteristic streak. Obviortsly basic rocks, they 
present, as I have said, a close external resemblance to many of the lavas of 
the Lower Old Eed Sandstone and Carboniferous districts of Scotland. 
From their chemical composition and microscopic structure they may be 
most appropriately ranged among the diabases. The analyses of two of 
the most conspicuous diabases of this class in the district, those of lihosson 
(VII.) and Clegyr Foig (VIII.), by Mr. J. S. Grant AVilson, are shown in 
the following table : — 
SiOa 
AI 3 O 3 
FeaOs 
FeO 
MnO 
i 
CaO ! MgO 
KoO 
Na^O 
H 2 O and 
Loss on 
Ignition. 
Insoluble 
Residue. 
Total. 
Specific 
Gravity. 
YU. 
45-92 
18-16 
1-18 
9-27 
0-19 
7-19 10-07 
1-78 
2-12 
4-22 
0-04 
100-14 
2-96 
VIII. 
45-38 
16-62 
4-06 
8-63 
0-14 
8-19 9-41 
0-71 
2-20 
4-34 
0-08 
99-76 
2-99 
The two rocks here analyzed, likewise that from the crag south of Castell 
and that from the cliffs at the southern end of the promontory between 
Ilamsey Sound and Pen-y-foel, show under the microscope a general similarity 
of composition and structure. They present a variable quantity of a base, 
which under a ^ objective is resolved into ill-defined coalescent globulites 
*md fibre-like bodies, that remain dark wlien rotated between crossed 
nicols. In some varieties, as in part of Ilhosson Crag, the base is nearly 
lost in the crowd of crystalline constituents ; in others, as in the crag south 
of Castell, it forms a large part of the whole mass, and may be seen in 
distinct spaces free from any crystalline particles. Through this base are 
diffused, in vast numbers, irregularly-shaped grains of augite, seldom show- 
ing idiomorphic forms. These grains, or granules, may perhaps average 
about 0-003 inch in diameter. Plagioclase is generally hardly to be recog- 
nized, though here and there a crystal with characteristic twinning may be 
detected in the base. Magnetite occurs abundantly — its minute octahedra, 
with their peculiar colour and lustre, being apparent with reflected light on 
the fresher specimens, though apt to be lost as diffused ferruginous blotches 
in the more decomposed varieties. But perhaps the most remarkable in- 
