302 HB, HICKS ON THE PRE=CAMBRIAN ROCKS IN 
this time with much success. It seemed but natural now also, after 
what we had seen in the other areas, that we should, in the first 
place, attempt to discover some base-line or axis from which to start ; 
and we were fortunate in discovering such a line near Ty-Croes, 
a railway-station on the Holyhead line. A little to the west of this 
point, in the Geological Survey maps, a large patch is coloured as 
granite running in a direction nearly N.E. and 8.W., and with a 
width in some places of between two and three miles. On examining 
this we soon found that we had here an almost identical rock with that 
found at Twt Hill, Caernarvon, at Rhos Hirwain, and at St. David’s, 
and that we had in reality come upon what would in all likelihood 
prove to be another Dimetian axis. This was very satisfactory ; and 
the microscopical examination of the rocks forming this ridge sub- 
sequently made has fully borne out the conclusions then arrived at 
in the field (vide Appendix, No. VIL.). 
We were fortunate also in finding on the roadside, and in a quarry 
near some cottages about half a mile W.N.W. of Ty-Croes, the 
actual junction between the Dimetian (or granitoid) rocks and the 
adjoining ones. These last appeared brecciated at this point and 
seemed to contain fragments of the granitoid rocks. Tracing these 
backwards towards Ty-Croes, this brecciated character was soon 
lost, and compact greenish-grey rocks of the hilleflinta type appeared 
to rest upon them in natural succession. To the east of Ty-Croes 
micaceous and chloritic schists again lie upon the halleflinta group, 
and dip away from the latter towards the 8.H. So that in this 
neighbourhood, and within a small compass, we meet with rocks 
belonging to three distinct formations, and in the order of suc- 
cession indicated already in other areas. The granitoid gneiss and 
more compact granitoid rocks compose the Dimetian axis. Flank- 
ing this along the eastern edge are found the breccias and hiille- 
flintas of the Arvonian; and beyond and resting probably uncon- 
formably upon these the micaceous and chloritic schists of the 
Pebidian. Professor Bonney, who subsequently examined this neigh- 
bourhood, has kindly furnished me with notes upon it, which are of 
great importance as confirmatory of our conclusions, and also as 
adding new and valuable supplementary information. They refer 
chiefly to the area west and south of the point indicated above, 
where I consider the junction of the Dimetian with the Arvonian 
occurs, and are important in showing the presence of bands of chloritic 
schistose rocks associated with the granitoid rocks here, similar in a 
marked degree to those described by me in the Dimetian at St. David's. 
The following are the notes by Professor Bonney :— 
“Tn a shallow cutting just south of Ty-Croes station (on railway) 
is a hard greyish bedded rock, like a fine felspathic grit or mudstone 
much altered. Walked along highroad towards Llanfaelog, passing 
over schist. At Felinbont the rock is a greenish grey, rather cal- 
careous, micaceous schist, of the general type so common in Anglesey, 
the foliation-planes striking N.H. and §8.W. with dip to 8.H. Came 
on to the first of the two ‘granite’ promontories of map. ‘The 
rock not a granite, but a granitoid rock like that of Twt Hill (Ap- 
