278 AN ENQUIRY INTO THE NATURE OF [Ch. XIII- 



position of these strata, next the ranges of trap, be attributable 

 to the protrusion of the latter, whence this want of uniformity 

 in the results? sometimes the strata are perpendicular on 

 both sides, sometimes only on one ; or, they are inclined on 

 both sides in the same or different directions : nor must it be 

 omitted to be remarked, that, owing to the curvatures of the 

 strata, the same discordant dips may be seen where no trap is 

 present ; as also holds good with respect to contortions, which 

 sometimes occur, and at other times not in the vicinity of 

 trap ; moreover, this protruded trap itself is sometimes com- 

 posed of strata equally elevated, for " the southern part of 

 Skomer Island consists of stratified greenstone, dipping at 

 about 48 to the S. E." Here the supposed cause and effect 

 are so confounded, that, even admitting the trap of Pembroke- 

 shire to be an igneous intrusive rock, it does not follow that 

 it has caused the elevation of the adjacent strata. 



But we must not omit to notice an instance of elevation of 

 a less doubtful character, which has been recorded by Lyell 

 in his description of the largest of the Cyclopian islets. " The 

 summit and northern sides," he says, " are formed of a mass 

 of stratified marl (creta), the laminae of which are occasionally 

 subdivided by thin arenaceous layers. These strata rest on 

 a mass of columnar lava, which appears to have forced itself 

 into, and to have heaved up, the stratified mass. This theory 

 of the intrusion of basalt is confirmed by the fact that, in 

 some places, the clay has been greatly altered and hardened 

 by the action of heat, and occasionally contorted in the most 

 extraordinary manner, the laminae not having been obliterated, 

 but, on the contrary, rendered much more conspicuous, by 

 the indurating process." * 



Now, this case appears to be a most conclusive example of 

 the elevation of strata by igneous rocks, inasmuch as the beds 

 are not only inclined, but are also said to be altered by the 

 protrusion of the lava in a state of incandescence. Even 



* Principles of Geology, vol. iii. p. 79. 



