Prof. T. G. Bonney— Welsh Pre-Cambrian Rocks. 301 
intrusive felsite.! Also, admitting the identity of the Carez Goch 
conglomerate and grit with that at Port Dinorwic, there is strong 
evidence to show the improbability of its being later than the felsite. 
We are here within a very short distance of the edge of a great 
mass of the latter rock, yet no trace of it even with the microscope 
can be found in the former. Again, we find this conglomerate at 
the back of a row of white cottages (at Tan-y-maes), and pick it up in 
the path in front. This path then descends a rocky step to the side 
of a little stream. The main mass of the felsite is to be seen at 
Tan-y-perthi, a very short distance to the N.N.E. The rock, where the 
path descends, resembles the granitoid series, but on examination 
proves to be a spherulitic quartz-felsite. Thus, either the conglome- 
rate is dropped into a notch in the felsite, and one portion of this 
has assumed a very unusual structure, or the spherulitic felsite is 
intrusive. The annexed diagram will give a general idea of the 
Rough Diagram of relation of rocks between Tan-y-maes and Tan-y-perthi. 
emeeenen----3 0 
A Felsite. B Spherulitie Felsite. 
C Exposure of Conglomerate and Grit. D Stream. 
relations of the rocks. The evidence is not indeed absolutely con- 
clusive, but, as it seems to me, the probability is so great, that the 
burden of proof lies on the other side.” 
Proceeding now to the district on the north-eastern edge of this 
great mass of felsite, I described—in ascending order—(1) a 
purplish felsitic grit; (2) a series of slates and beds of agglomerate 
or breccia (Tairffynnon), which terminate with a peculiar breccia, 
containing much purple slate, which is seen near Cae Seri; then (8) 
a slaty and agglomeratic series (Bangor Beds of Prof. Hughes). 
(1) and (2), speaking in general terms, Prof. Hughes suppresses, 
regarding every grit or rock with felsite fragments as merely Cambrian 
conglomerate repeated by faults. I confine my remarks at present 
mainly to the district on the eastern side of the fault, which runs 
near the line of the Bangor-Carnarvon road. First, then, the above 
grit—which we trace along the edge of the felsite near Brithdir Farm 
down towards the valley, pick up on the other side of the fault by 
the road side, again near a farm-house higher up the hill, and in the 
road by Beulah chapel—in all these places is in close sequence with 
the felsite, and is composed of detritus from it. We observe a 
parallel case in the grit overlying the yellowish felsite on Conway 
Mountain, and at Diganwy. Is it not, then, rather a singular coinci- 
dence that, if this purplish grit be dropped down by a great fault, 
cutting out all the Bangor series, it should be so completely made 
up of the detritus of the felsite, and should come for so great a 
! Discovered by Mr. Tawney. 
* Certain reasons connected with the proximity of the cottages make it difficult, 
especially in summer time, to examine the ground inch by inch. 
