490 EEV. J. F. BLAKE ON THE 



the relations of these several rocks with those that obtain further 

 to the north, they are not the same, and we must therefore regard 

 the more special types as accidents in the series. There would 

 appear to have been two volcanic foci — one near Llangefni, and the 

 other near this spot, which latter may be connected with the area, 

 to be subsequently described, on the other side of Malldraeth Bay. 



I gather, therefore, that we have in this district a somewhat 

 similar development to that which occurs between Yalley and Pen- 

 bryn-yr-Eglwys, with local differences, of which the principal is the 

 absence of any granite amongst the ashes or pelites of the district. 

 I can see no reason for widely separating any part of the rocks on 

 the east side of the fault from their neighbours. 



The Westeen Region. — We can now consider the area which lies 

 on the western side of the dividing line ; and as the main feature of 

 this is the granite, and our view of the stratigraphy must necessarily 

 depend on the nature of this rock, and as it has lately been denied 

 that it is granite, but asserted to be a metamorphic sedimentary 

 rock, it is necessary, in the first place, to bring field-observations 

 to prove that it is intrusive. 



a. The Intrusive Occurrence of the Granite. — The proofs of this are 

 so many that their enumeration threatens to be tedious. To adduce 

 them all, however, is a necessity forced on us by recent literature. 

 Commencing at the south-western end, we find the eastern side of 

 Llyn Faelog coloured as granite. Here there are a number of 

 bosses of rock protruding from the ground. On examining several 

 of these, we find the two sides composed of different material ; ou 

 the one side is the pelite, on the other the granite. The passage 

 from one into the other is obscure, and from this locality alone it 

 would be open to any one to say that they here recognized the gradual 

 metamorphosis of the sedimentary rock into granitoidite. But 

 we can also interpret the phenomenon as an absorption by the 

 granite of the neighbouring rock, in circumstances under which the 

 former was kept long heated in contact with the latter. On the 

 coast at Porth-ceryg-defaid the pelite again contains patches of the 

 granite, but they are both brecciated. At a small promontory be- 

 tween this and Porth-y-ly-wod, however, we get the first clear proof 

 of intrusion. Here on the shore is a low boss of rock, with all its 

 surface bare (see fig. 8). The pelite is more or less orientated in 

 the direction of the sea. Into this, and running in the same 

 general direction, but in more than one place crossing its structural 

 lines, is a tongue of granite from 3 to 4 feet wide, which connects the 

 two parts of the boss, and finally disappears with it beneath the 

 sand. Hard by to the east is another smaller string of granite not 

 quite parallel to the larger tongue, and also crossing the lines of the 

 pelite, but dying out before reaching the second half of the boss, 

 and the rocks in both parts are perfectly distinct. The granite, in 

 structure, is seen to be much brecciated, doubtless in the subsequent 

 compressions of the rocks ; but in the unbroken parts it presents a 

 typical holocrystalline granitic arrangement, with largish patches 

 of original white mica. 



