BRITISH TRAP ROCKS SHOWN TO BE VOLCANIC. 



73 



many of these great eruptions and dislocations are intimately associated with other 

 phenomena not less indicative of change. Thus within modern times, for example, 

 irregular-shaped and narrow wedges of volcanic matter have crossed through the beds 

 of ashes, scoriae, and lava which envelope and form the sides of Vesuvius ; and in like 

 manner, similar masses (the dykes of geologists) are inferred to have penetrated in 

 ancient times the solid strata of sandstone and limestone ; the latter where they are in 

 contact with the intrusive rock, being often changed into a crystalline or highly indurated 

 state. He further perceives that veins filled with simple minerals, frequently radiate 

 through the sedimentary rocks where they are contiguous to such centres of eruption ; 

 and thus he supplies the chemical philosopher with data, which at some future day 

 may materially aid in solving the difficult problem of the origin of metallic veins ; at 

 least the geologist shows the extent to which such veins may have been originally de- 

 pendent on volcanic action. 



In our country we have no traces of subaerial volcanos ; no craters like those of 

 Auvergne, with pumice and scorise adherent to their sides, or streams of lava traceable 

 to their mouths ; nor have we volcanic islands rising from the bed of the sea : our only 

 present memento of the existence of volcanic action beneath us, consisting in very slight 

 shocks of earthquakes. On the other hand, however, Great Britain is rich in rocks 

 possessing many mineral characters in common with volcanic products, being com- 

 posed like them principally of felspar, augite, hornblende, and other minerals, and 

 connected with them by so many gradual shades and links, that nearly all naturalists 

 are now agreed that such rocks are the result of heat and fire. These are the syenites, 

 porphyries, greenstones, clinkstones, and basalts, which have been forced up as molten 

 matter through the submarine accumulations now constituting our sedimentary deposits 1 . 

 Hutton, Playfair, and Hall were the first British geologists who brought the prominent 

 features of such phenomena under the test of observation and experiment ; and clearly 

 proved that all trap dykes were of igneous origin, and had been intruded among pre- 

 existing strata ; for it was shown, that as these rocks must have been erupted under 

 the pressure of an ocean, so ought they, instead of being light and porous, to be hard, 

 compact, and heavy, as we now find them. Obvious as these views may now appear, 

 they required much subsequent illustration and support, before they could obtain that 

 general assent to which they were entitled ; but to use the language of a sound observer, 

 " Nature fortunately remains more stable than prejudice*," and few indeed are those 



1 It was this very intermixture of rocks having an igneous aspect with others which had been clearly formed 

 under the sea, which led to those disputes about aqueous and igneous origin that now appear so trivial ; the 

 advocates of the Neptunian theory contending that all trap rocks were precipitates formed under water, because 

 when compared with lavas they were found to be more compact ! Yet, as Mr. Lyell has well observed, " The 

 terms of comparison were imperfect, for one set of rocks formed almost entirely under water was contrasted 

 with another which had cooled under the open air." (Principles of Geology, vol. iv. p. 353.) 



2 Bakewell, Travels in the Tarentaise, Alps Auvergne, vol. ii. p. 295. 



