Yol. 68.] PEE-CAMBRIAN AND CAMBRIAN OF PEMBROKESHIRE. 375 



Before proceeding to a description, of the results of our survey, 

 reference may be made to tlie principal works bearing on the district 

 and to the chief points established by previous writers. 



{a) Historical Review. 



The first reference to the geology of this district is in the 



* Silurian System' (1),^ published in 1839, in which Sir Eoderick 

 Murchison mentions the Cambrian rocks at Cwm Mawr, Newgale. 

 He does not refer to the igneous rocks of the neighbourhood ; but 

 he detected the faulted nature of the junction between the Cambrian 

 and the Carboniferous (Millstone Grit), at I^ewgale, although 

 (p. 374) he says that 



'Near Brawdy the culm field rests upon, and has the appearance of 

 graduating into the Cambrian System.' 



The first mention of the igneous rocks of this old ridge was not made 

 until some years later (in 1842), when Sir Andrew Ramsay drew up 

 a manuscript report on the geology of Pembrokeshire (2). He had 

 noted the variation in texture of the granitic rocks, but regarded 

 the rhyolitic rocks of Ehindaston and Pointz Castle as fine-grained 

 modifications of the same. With regard to the granite, he says 

 that 



* the ground is so much obscured that it is impossible to see the manner in 

 which the granite affects the strata through which it is intruded, but from 

 various evidences I consider it probable that it shoots into veins into the 

 stratified rocks in a manner similar to the granite of Arran.' 



In 1845 when the results of the first official geological survey (3) 

 were published in Sheet 40 of the map, the main outlines of 

 the pre-Cambrian rocks were indicated ; the rocks of Brimaston, 

 Brawdy, and Pointz Castle were coloured as 'granite,' and those 

 of Ehindaston as ' greenstone.' The Cambrian beds were not difi'er- 

 entiated by colour, but the words ' Purple Beds ' indicated their 

 presence near Trefgarn Owen and Eoch Bridge, respectively on 

 the northern and southern sides of the pre-Cambrian axis. In the 

 following year (1846), Sir Henry De la Beche (5) gave a brief 

 description of the granite between St. Lawrence and Brawdy, and 

 remarked that towards its margin it passes into a rock similar to a 

 Cornish elvan. He states that 



* this granite . . , seems to have altered the stratified rocks in contact with it in 

 many places, but the date of its protrusion is uncertain.' 



Soon after this W. T. Aveline spent some time in the district 

 revising the geological map, preparatory to the issue of a new 

 edition (7) ; and one of his notebooks (6), preserved at the 

 Geological Survey Office, contains much valuable information. His 

 chief work was among the Lower Cambrian rocks which were sub- 

 sequently coloured on the map, and of which the most important 



^ These numerals refer to the Bibliography on p. 377. 



Q. J. G. S. No. 271. ' 2 e 



