118 Dr. ©. Callaway—Pre-Cambrian Geology of Anglesey. 
amorphous type. At many points round Llechyn farwy and 
Llandrygarn, as will be shown in detail, the granitoidite is clearly 
interstratified with schist, and passes into it both horizontally and 
vertically. As confirming the identity of the Anglesey and Twt 
Hill granitoidite, the following fact is of importance. Messrs. 
Bonney and Houghton! have detected at Twt Hill a passage between 
the granitoidite and a quartzose conglomerate with a south-east dip. 
T have visited this section, and, having examined the rock inch by 
inch, I can entirely confirm their identification. There are no 
signs of a fault between the granitoidite and the conglomerate, and 
the transition between the two is gradual and unbroken. I have 
had the good fortune to discover this identical conglomerate in 
Anglesey. It is exposed in two quarries near Nebo, two miles 
south-east of Amiwch, dipping to the north-west at a high angle. 
Lithologically it is perfectly indistinguishable from the Twt Hill 
rock; the quartz has the same glazed surface, both conglomerates 
contain disseminated crystals of cubic pyrites, and are tinged with 
the same dingy purple colour. I could find no granitoid rock in 
these quarries, but the ordinary granitoidite occurs on about the 
same strike, one-third of a mile to the north-east. That this 
conglomerate is not Cambrian or Ordovician? is proved by the fact 
that in these quarries black Ordovician (Carodoc or older) shales 
rest upon its upturned edges. It is also associated with bands of 
quartzose grit as in the Twt Hill locality. 
The sedimentary origin of the granitoidite being proved, it remains 
to demonstrate that it is of Pre-Cambrian age. The unconformity 
between the Ordovician group and the conglomerate of Nebo, 
just noticed, might perhaps be considered decisive of the question. 
If this be denied, we are thrown back upon the hypothesis 
that, between the Cambrian and the Caradoc periods, the older 
series was metamorphosed into granitoidite and schist, tilted up at 
a high angle, and largely denuded. This, I presume, no geologist 
will maintain. The evidence from included fragments also tends in 
the same direction. Prof. Bonney* noticed a pebble of the grani- 
toidite in the Cambrian conglomerate of Llyn Padarn, and I have 
since observed that such inclosures are not uncommon. Prof. 
Ramsay furnishes similar testimony. He describes the “Silurian” 
conglomerate of Anglesey in the following terms :—* The pebbles 
of the conglomerate are sometimes six or eight inches in diameter, 
and, taken from different places at random, they consist of white 
quartz, grey quartz-rock, mica-slate, green schist, jasper, purple 
slate, a granitic rock of quartz and felspar with sometimes a little 
mica, blue felspathic trap, dark green hornblende rock, and chlorite.” 
I have seen this conglomerate in several localities, and could in all 
material points confirm, were it necessary, the above description. All 
of the varieties named occur in the metamorphic groups of Anglesey. 
: Quart. Journ. Geol. Soc. vol. xxxv. p. 322. Ms 
- A have adopted Mr. Lapworth’s name (Grou. Mac. Jan. 1879) for the rocks 
called “ Lower Silurian,” that is, the groups from the Arenig to the Caradoc 
inclusive. a Quarles eels 
* Geology of North Wales, p. “i = ourn. Geol. Soe. vol. xxxy. p. 316. 
