502 EEV. J-. F. BLAKE ON THE 



in the island, the base of which is now exposed by denudation ; as 

 its central mass there is a great variety of granite-rocks, which are 

 intrusive in many cases into those around them. Some of these 

 surrounding rocks may be absorbed by the granite, and some may 

 have been produced by its brecciation. But anterior to the granite, 

 and intruded into by it, is a highly foliated diorite, which now 

 occurs in more or less isolated areas. There are no agglomerates 

 in this region, but the ashes are everywhere minute, whether in 

 the more acid form, as halleflintas, or the less acid, as pelites ; and 

 they are much impregnated with secondary minerals. Calcareous 

 bands are also present among them. Some of the rocks found 

 exposed between the masses of granite are of obscure origin, and by 

 their thoroughly crystalline and foHated character, and yet their 

 distinctness from the grey gneiss, suggest the possibility of their 

 being truly Archaean. With very few exceptions the whole of the 

 rocks in this district are composed of finer ingredients, and are 

 more metamorphosed than those of the western district. They are 

 overlain in all but their eastern margin by the basal rocks of the 

 Ordovician, which everywhere contain their fragments. In spite 

 of the separation by faulting, and the apparent unconformability 

 at Bodafon, there is a unity of character and intimate association 

 amongst all the parts that prevents us separating any part more 

 widely from another than as earlier and later developments of one 

 great system. 



The District "West op Teaeth Dulas. 



This is a small and entirely isolated area which, from its 

 proximity to Bodafon mountain, is naturally described in the present 

 order. It figures on the Survey map as " altered Silurian," though 

 no reason for this is assigned in the memoir. Dr. Eoberts has 

 shown that to the west of Pen-Ion a conglomerate which he calls 

 Cambrian, but which is really the Ordovician basement-bed, lies 

 over the granite. This is, in fact, carried on to the top of the granite 

 area somewhat as marked in the Survey map, and is continued all 

 along the northern boundary as far as Llaneiddog ; on the southern 

 side grits are seen at the junction in the road from Worn, and also 

 in the stream that runs out at Traeth Dulas. It is thus surrounded 

 by Ordovician basement-beds, and there can be no doubt of its 

 being Pre-Ordovician. The conglomerate, however, which is in two 

 or three bands, is not composed of the immediately underlying 

 rock, but of quartz and jasper. 



The district is not noticed by Dr. Callaway, and the sole descrip- 

 tion by Sir A. Eamsay is as follows : — " The granite is necessarily 

 mapped chiefly as one mass with several smaller patches, piercing 

 the associated highly metamorphic gneissic mica- schist on the south ; 

 but in reality they are inseparable from each other, so intimately do 

 they seem to be interlaced." 



This old island is, in fact, a mass of granite intruding into what 

 appears to represent the grey gneiss^ We have seen the latter 



