404 



DISTINCTIONS BETWEEN BEDDED AND INTRUSIVE TRAP. 



hand specimens it could not be distinguished from many sedimentary rocks. It performs, however, 

 most clearly the part of an intrusive rock, since it has not only flowed in one part in bulging irre- 

 gular streams over dislocated and angular portions of sandstone, but also throws up other strata of 

 sandstone into a vertical position, which in contact with the trap conglomerate are almost in the 

 state of quartz rock as represented in this wood-cut. 



83. 



Fault. 



a. Old Red Sandstone. a*. Altered Old Red. b. Trap conglomerate. 



The direction impressed upon the associated strata by the eruption of this band of trap, is well 

 exposed in the cliffs, a few yards south of the trappean conglomerate, the Old Red Sandstone having 

 a strike from E.S.E. to W.N.W. 



The surface or agricultural character of these ridges of trap of Johnston and Benton Castle con- 

 sists for the most part of absorbent, sandy soils, and where the greenstone is much decomposed, 

 they resemble those sandy and gritty tracts in France derived from what Dolomieu termed " Granite 

 pourri." 



Trap of Skomer Island and Marloes. 



No part of Pembrokeshire, which fell under my notice, offers such clear examples of both 

 the stratified and unstratified trap rocks, as the Island of Skomer and the adjacent pro- 

 montories of the mainland, which form the western end of Broad Sound. When viewed 

 from a distance, the outline of Skomer is most remarkable, presenting a number of steps 

 or ledges, which unlike the prevailing forms of trap are not horizontal, but inclined at 

 angles of 35° and 40° to the S.S.E., thus appearing to dip in conformable stratification 

 beneath the Silurian System of Marloes Bay. (See vignettes, p. 389 and 392 \) 



When closely inspected, these stratified masses are found to consist of purely crystalline green- 

 stone, dark green, granular felspar rock, and felspar breccia or conglomerate, which alternate 

 conformably in thick, parallel masses, with regularly stratified, purple, green, and yellow sandstone 

 and schist of the Upper Cambrian System. Such alternations are displayed in North, South, and 

 Wick Havens, in each of which the powerful currents of the sea, which set in at these extreme 

 points, have worn the softer portions of the rock into coves, the only spots along the rugged coasts 

 of the island where a boat can land. The stratified masses of greenstone and felspar conglomerate 

 are so numerous, and alternate so equably and frequently with sandstone and schist, that I con- 

 sider they must have been evolved at intervals from volcanic fissures at the bottom of the sea, during 

 the accumulation of the sand, pebbles, and mud, with which they are associated, and into which 

 there are occasionally such close passages, that it is most difficult to say where the trap rock termi- 

 nates. For further description of similar contemporaneous trap rocks, and for explanations of the 

 mode in which they have been formed beneath the sea, the reader is referred to chapters 5, 19, 22, 



1 Both these wood-cuts are taken from sketches by Mr. Francis Leach. 



