Prof. Bonney—Mierescopie Structure of Pre-Oambrian Rocks. 125 
age is rendered still more improbable by the fact that they are mainly 
gneissic. The discussion of their true age does not belong to the 
present paper. 
In the last section, we saw, still more clearly than at Pen y 
carnisiog, the intimate association of volcanic rocks with the grani- 
toid group. In further illustration of this point, and of the 
occurrence of hilleflinta in the Dimetian, I will call attention to two 
more localities. 
One mile north of the last section. between Ty newydd and 
Tyn rhos, quartz-felsite (Note 7, p. 126) is seen at more than one 
point, and in each case it passes by an imperceptible transition into 
the ordinary granitoidite (Note 8, p. 126). 
Still further to the north, between Gwyndy and Pen y bone, a 
thick band of hiilleflinta is exposed in a road section. This bed 
is towards the base of the Dimetian, since, towards Gwyndy, the 
latter passes down into the dark gneiss. 
Summary. 
1. The granitoid (Dimetian) rocks of Anglesey pass down into 
an anticlinal of dark gneiss (above) and grey gneiss (below). The 
gneiss is also seen in the Menai anticlinal. 
2. Low down in the granitoid series are bands of felsite, hiille- 
flinta, and felspathic breccia, which can lay no claim to the rank of 
a distinct group (Arvonian). 
3. The schistose series at Ty Croes is brought into contact with 
the principal quartz-felsite and hilleflinta band by a fault, and there 
is, therefore, no proof, from this section, that the schists are of 
Pebidian age. 
VII.—Nore on rue Microscopic Structure oF some Pre-CAaMBRIAN 
Rooks. 
By Prof. T. G. Bonney, M.A., F.R.S., Sec. G.S. 
OME repetition may be avoided in this note by roughly grouping 
S the specimens, which have been forwarded to me by Dr. 
Callaway, as (A) Felsitic Rocks, or those resembling felstones ; (B) 
Granitoid and Gneissic Rocks; and (C) Dioritic Rocks. The 
numbers attached correspond with the reference-numbers in Dr. 
Callaway’s paper. I may mention that in examining the slides I was 
ignorant of their bearing on the views propounded by the author, 
and, in some cases, of the exact locality of the specimens. 
(A).—Fetsrtic Rocks (4-7). 
These are compact generally light-coloured rocks, breaking with a 
rather sharp fracture, and weathering to a pale, whitish tint. The 
ground-mass of each consists chiefly of microcrystalline quartz and 
felspar, of very indefinite outline and arrangement, in which occur 
some larger grains of the same minerals, with more or less opacite 
and other microlithic minerals. The ground-mass of (4) exhibits a 
wavy graphitic or almost arborescent structure not uncommon in 
felsites.. The larger quartz and felspar grains are generally ragged 
1 Described by myself, Gzou, Maa. 1877, Dee. II. Vol. IV. p. 508. 
