2 Mr. De la Beche on the Geology of Southern Pembrokeshire. 



by the tortuous coast which extends from St, Bride's Bay on the west to 

 Caermarthen Bay on the east^ and by the shores of Milford Haven which tra- 

 verses the centre of the district, afford remarkable facilities for investigating 

 the structure of the country. 



The surface of the district^ in general, is exceedingly undulated : few of the 

 ranges, however, are striking, nor are there any of very considerable height. 

 The country near the shores of Milford Haven is often well wooded ; but in 

 other parts it is almost bare of timber, and particularly to the north-west of 

 Haverfordwest. 



Of the formations occurring within the district I shall treat in the following 



order : 



1. Trap*. 



2. Greywacke. 



3. Old red Sandstone. 



4. Carboniferous, or Mountain Limestone. 



5. Coal-measures. 



§ 1.— Trap. 

 The rocks which I mean to comprehend under this name, are very abun- 

 dant in the district. They form in several cases insulated hills, and occasion- 

 ally continuous ridges of some extent. Their appearance on the surface is 

 usually distinguished by craggy piles of rock, such as in Devonshire are called 

 Tors, but in Pembrokeshire and other parts of Wales are termed Cams. The 

 best characterized varieties of trap consist of distinctly crystallized aggregates 

 of felspar, quartz, and hornblende, forming syenite or greenstone, according 

 as the first or last of these ingredients predominates. The former rock is of 

 much rarer occurrence than the latter. Augite has not hitherto been dis- 

 covered in these coarsely crystallized aggregates. 



* I have placed the trap at the head of the formations of the district, not as being invariably 

 the lowest in the order of position, nor as being for that reason regarded by me as the most 

 ancient ; but because it is liable to occur in any part whatever of the series, and therefore is 

 frequently found to occupy the lowest place. In the sequel I shall have occasion to notice many 

 appearances which strongly favour the notion that the trap has been forcibly intruded amongst 

 the other rocks at a period subsequent to their consolidation. In applying therefore, in the 

 courseof the present memoir, the expressions "strata," "stratified," &c. to trap, itisby no means 

 my intention to discountenance the notion above alluded to, much less to signify my adoption of 

 any other in opposition to the former. I only mean to imply by these expressions, that there is 

 a parallelism of texture in the trap, which it has in common with a contiguous rock belonging to 

 some other formation ; and that this texture is also parallel to the common surface of separation 

 between the trap and that other rock. 



