280 



LYELL'S ELEMENTS OF GEOLOGY. 



Silurian Volcanic Rocks Cambrian Volcanic Rocks. 



ry and amygdaloid, the kernels of the latter being sometimes 

 calcareous, often chalcedonic, and forming beautiful agates. We 

 meet also with claystone, clinkstone, greenstone, compact felspar, 

 and tuff Some of these rocks flowed as lavas over the bottom 

 of the sea, and enveloped quartz pebbles which were lying there, 

 so as to form conglomerates with a base of greenstone, as is 

 seen in Lumley Den, in the Sidlaw Hills. On either side of 

 the axis of this chain of hills (see section, p. 65.), the beds of 

 massive trap, and the tuffs composed of volcanic sand and ashes, 

 dip regularly to the south-east or north-west, conformably with 

 the shales and sandstones. 



Dr. Fleming has observed similar trap rocks in the old red 

 sandstone of northern Fifeshire, where they are covered imme- 

 diately by the yellow sandstone which forms the base of the 

 mountain limestone and coal-measures. 



Silurian period. — It appears from the investigations of Mr. 

 Murchison in Shropshire, that when the lower Silurian strata of 

 that county were accumulating, there were frequent volcanic 

 eruptions beneath the sea ; and the ashes and scorise then ejected, 

 gave rise to a peculiar kind of tufaceous sandstone or grit, dis- 

 similar to the other rocks of the Silurian series, and only ob- 

 servable in places where syenitic and other trap rocks protrude.* 

 These tuffs occur on the flanks of the Wrekin and Caer Cara- 

 doc, and contain Silurian fossils, such as casts of encrinites, tri- 

 lobites and mollusca. Although fossiliferous, the stone resembles 

 a sandy claystone of the trap family. 



Thin layers of trap, only a few inches thick, alternate, in 

 some parts of Shropshire and Montgomeryshire, with sedimen- 

 tary strata of the lower Silurian system. This trap consists of 

 slaty porphyry and granular felspar rock, the beds being tra- 

 versed by joints like those in the associated sandstone, limestone, 

 and shale, and having the same strike and dip.:}: 



In Radnorshire, there is an example of twelve bands of stra- 

 tified trap alternating with Silurian schists and flagstones in a 

 thickness of 350 feet. The bedded traps consist of felspar-por- 

 phyry, clinkstone, and other varieties ; and the interposed Llan- 

 deilo flags are of sandstone and shale, with trilobites and grap- 

 tolites.§ 



Cambrian volcanic rocks. — In Pembrokeshire stratified, 

 greenstone, felspar-rock, and a breccia containing fragments of 

 trap, alternate conformably in thick parallel masses with regu- 

 larly stratified sandstone and schist of the upper Cambrian sys- 



* Murchison, Silurian System, &c. p. 230. 

 t Ibid. X Ibid. p. 272. 



$Ibid. p. 325. 



