382 ME. H. H. THOMAS AND PEOP. 0. T. JONES ON THE [Sept. I912, 



Passing now to a consideration of the diorite of Knaveston, the 

 determination of its relative age is a matter of considerable 

 difficulty ; for, although it appears to be in natural contact with 

 granitic rocks on its western side, no junction is visible; it is 

 faulted on the north against granite, while on the east and in the 

 immediate south the ground is so obscure and drift-covered that it 

 is unsafe to base any far-reaching conclusions on the evidence 

 available. It is possible that the diorite is quite a small intrusion, 

 smaller than is indicated on the map, and may be of later date 

 than the granite with which it is in contact. This, however, 

 would be contrary to the order usually observed in the sequence 

 of plutonic intrusions, although it would be analogous to that 

 presented by the dioritic dyke-phase in the supposed pre-Cambrian 

 of South Pembrokeshire.^ 



Doubt may be thrown on the presence of a dyke-phase of the 

 Dimetian in this district, but appearances are in favour of its 

 existence. Several basic dykes have been detected in the Dimetian 

 and Pebidian ; and although, as at Brawdy and Silver Hill, they 

 strike obliquely to the outcrop of the Cambrian rocks, they cannot 

 be traced from the older into the younger system. We have no 

 reason to believe that in these cases the base of the Cambrian is 

 faulted against the older rocks — in fact, the field evidence is against 

 such an explanation of the sudden ending-off of the dyke-rocks. 



(i) The Granite. (D\) 



The granite of this region has been mentioned at various times 

 and brief descriptions given by several writers, but its correct limits 

 were not defined on any geological map. 



It occurs in several isolated patches, and forms a relatively small 

 portion of the pre-Cambrian complex. The areas in which it comes 

 to the surface are Brimaston and Brawdy, but in neither of these 

 districts is it well-exposed. It has weathered to a soft sand, and in 

 many places, as near Brawdy Vicarage, has been dug to a depth of 

 several feet. 



Sir Andrew Ramsay (2) noted that 



* owing to the readiness with which it decomposes, particularly when large- 

 grained, it is I'are to find it in solid masses in place. The large-grained variety- 

 decomposes most easily, and in some places where sand-pits have been dug at 

 the depth of six or eight feet from the surface, it is still easily dug out with a 

 shovel.' 



In the neighbourhood of Brimaston the granite is best observed 

 on the somewhat steep bank that flanks the northern side of [N'ant- 

 y-coy Brook. The easternmost patch, which is faulted off from the 

 main mass, is exposed in a small quarry overlooking the stream, 

 and consists of a fine-grained non-porphyritic rock of a yellow to 

 buff colour, speckled with flakes of decomposed biotite. A darker, 



^ O. T. Jones, in ' The Geology of the Country around Haverfordwest 'Mem. 

 Geol. Surv. Explau. Sheet 228 ; in the press. 



