PEE-CAMBRIAN ROCKS OF ST. DAVID S. 



263 



occupied by the two kinds of rock are depicted on the geological 

 map of South Pembrokeshire accompan)dng this memoir. 



In the ' Silurian System ' (1839) Sir Roderick Murchison inserted 

 a brief description of the trap-rocks of Pembrokeshire, in which he 

 spoke of them as consisting of two classes, " viz., stratified masses 

 alternating with sedimentary deposits, and amorphous masses which 

 have burst through the strata." Among the older trap-rocks he 

 distinguished " thick-bedded coarse felspathic conglomerates, contain- 

 ing fragments of schist and slate, which range from north-east to 

 south-west in allinement with the other ridges of amorphous trap." 

 These words would seem to imply that he had recognized the 

 presence of true tuffs or fragmental igneous rocks in that region. 

 He also noticed proofs of the intrusive character of some of the 

 amorphous masses ; for in the district of Roche Castle, Trefgarn, and 

 Ambleston he found that " the intrusion of this [igneous] rock has 

 produced a powerful effect upon the adjacent strata, particularly on 

 those masses inclosed between the forks of trap in the gorge of the 

 river at Trafgarn, where the red and green sandstones are converted 

 into a brittle, siliceous substance resembling the ribbon jasper of 

 mineralogists " (pp. 401, 402)*. 



In the first volume of the ' Memoirs of the Geological Survey of 

 Great Britain/ De la Beche makes further reference to the geology 

 of Pembrokeshire, and in particular to some of the rocks which have 

 been the subject of more recent discussion. He refers to the rock 

 of Roche and adjoining districts as illustrating the remarkable 

 varieties of texture assumed by the same mass of igneous rock; and 

 he cites the granite between St. Lawrence and Brawdy, a few 

 miles to the east of St. David's, as presenting along its margin a 

 fine-grained variety, like a Cornish elvan, owing to more rapid 

 cooling, and as "certainly seeming to have altered the stratified 

 rocks in contact with it in many places "f. 



The Geological Survey of the St. David's district was begun as 

 far back as the year 1842, by Sir A. C. Ramsay, soon after he 

 joined the service. The Map was published in 1845, and was fol- 

 lowed by a sheet of Horizontal Sections across the district J. In 

 these publications all the igneous rocks are included in one colour 

 (green). But in the Horizontal Section the belt of country so 

 coloured, extending (on the map) from the sea at Porth-lisky to 

 beyond Llanhowell, is stated to be composed of " trap, in its strike of 

 various structure, — syenitic greenstone and felspathic volcanic ash"§. 



* The intrusive nature of the Trefgarn rocks was shown by Murchison in 

 1836 (Proc. Geol. Soc. vol. ii. p. 229). These rocks are included by Dr. Hicks 

 in bis "Arvonian" group. 



t Mem. Geol. Survey, vol. i. p. 230 (1816). Tbe belt of rocks here referred to 

 has been mapped by Dr. Hicks as belonging mainly to bis " Dimetian," but partly 

 to his " Arvonian " group. Quart. Journ. Geol. Soc. vol. xxxv. p. 287 (1879). 



J Sheet 40 of the Geological Survey Map of England and Wales, and Sheet 1 

 of tbe Horizontal Sections. 



§ Sir A. C Eamsay's field-maps, preserved among tbe official records of tbe 

 Survey, show that he not only recognized marked differences among the igneous 

 rocks, but that be mapped out tbe more important varieties. A MS. report 



x2 



