INTRUSIVE TRAP — DISLOCATING AND ALTERING SILURIAN ROCKS. 405 



23 and 26. These stratified masses of trap and greywacke are usually rent by faults and fissures, in 

 the proximity of the large amorphous masses of unstratified and amorphous trap. 



The intrusive trap cuts in vertical dykes through the bedded trap and sandstone, and rising into 

 the chief hills of the north-west of Skomer, also protrudes in irregular masses south of North 

 Haven, in Midland Isle, and both sides of Jack's Sound. Occupying the extremities of the mainland, 

 called the Hay's Point, Tuf ker Rock, and the Anvil, the same amorphous trap ranges along the coast 

 to the east by Hook Farm to Marloes Village, occupying two or three low tors within the promon- 

 tory. These rocks, which in this vicinity cut through all the deposits, from the Cambrian to the 

 Old Red System, including the Silurian strata, consist of highly crystalline greenstone of various 

 characters, some slightly porphyritic, and of compact felspar rock in great abundance, the latter 

 (as near the Hay's Point) being occasionally coated over with films of serpentine. This trap not 

 only contorts and dislocates the sandstone and schist with which it is in contact, but, as in num- 

 berless other cases, converts the former into granular quartz rock, of which a good example may 

 be seen at Martin's Haven. In this tract, while the stratified and alternating trap strikes from 

 E.N.E. to W.S.W., in conformity with the direction of the Cambrian and Silurian Systems ; the 

 intrusive trap bursts out at irregular points, traversing the above obliquely, or from east and by 

 south, to west and by north. 



This is clearly exemplified in the ridge of porphyritic greenstone, extending from 

 Marloes to Martin's Haven • and even where this rock appears in small or detached 

 points, it is accompanied by similar phenomena. The most striking example, perhaps, 

 is where the. little boss before alluded to, p. 393, juts through the Upper Silurian 

 Rocks in Marloes Bay, as represented in this wood-cut. (See Map.) 



The Three Chimnies. 



84. 



a. Strata of Sandstone, Shale. a*. Strata of Sandstone, Shale, altered in contact with Trap. 



b. Greenstone and Amygdaloid, &c. b*. Trap Conglomerate. 



The trap is there a crystalline greenstone, passing into a purple amygdaloid, with 

 kernels of green earth and veins of epidote, in a base of compact felspar. On the south 

 side, it graduates into an amorphous trap conglomerate, having a matrix of compact 

 felspar with included pebbles of quartz, some as large as a child's head. This mass 

 bulges out under twisted and altered strata of sandstone, whilst the chief body of the 

 crystalline trap, rises nearly to the top of the cliff (here about 150 feet high) and partially 

 overflows the ends of the beds, which are bent back and contorted, their strike being 

 changed from that of W.S.W. E.N.E. , which prevails throughout Marloes Bay, to west 

 and by north, east and by south. 



We thus see, that in South Pembroke, whether at Benton Castle or in Marloes Bay, 



3 E 



