700 SILURIAN YOLCANIC ROCKS. [Ch. XXXII. 



that country were accumulating, there were frequent volcanic erup- 

 tions beneath the sea ; and the ashes and scoriae then- ejected gave 

 rise to a peculiar kind of tufaceous sandstone or grit, dissimilar to 

 the other rocks of the Silurian series, and only observable in places 

 where syenitic and other trap rocks protrude. These tuffs occur on 

 the flanks of the Wrekin and Caer Caradoc, and contain Silurian 

 fossils, such as casts of encrinites, trilobites, and mollusca. Although 

 fossiliferous, the stone resembles' a sandy clay stone of the trap 

 family. * 



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

 parts of Shropshire and Montgomeryshire with sedimentary strata of 

 the lower Silurian system. This trap consists of slaty porphyry and 

 granular felspar rock, the beds being traversed by joints like those in 

 the associated sandstone, limestone, and shale, and having the same 

 strike and dip.f 



In Radnorshire there is an example of twelve bands of stratified 

 trap, alternating with Silurian schists and flagstones, in a thickness of 

 350 feet. The bedded traps consist of felspar porphyry, clinkstone, 

 and other varieties ; and the interposed Llandeilo flags are of sand- 

 stone and shale, with trilobites and graptolites.J 



The Snowdonian hills in Caernarvonshire consist in great part of 

 volcanic tuffs, the oldest of which are interstratified with the Bala 

 limestone and slate. There are some contemporaneous felspathic 

 lavas of this era, which, says Professor Ramsay, alter the slates on 

 which they repose, having doubtless been poured out over them in a 

 melted state, whereas the slates which overlie them having been sub- 

 sequently deposited after the lava had cooled and consolidated, have 

 entirely escaped alteration. But there are greenstones associated 

 with the same formation, which, although they are often conformable 

 to the slates, are in reality intrusive rocks. They alter the stratified 

 deposits both above and below them, and when traced to great dis- 

 tances, are sometimes seen to cut through the slates, and to send off 

 branches. Nevertheless, these greenstones appear to belong, like the 

 lavas, to the Lower Silurian period. 



Cambrian Volcanic Hocks. — The Lingula beds in North "Wales 

 have been described as 7000 feet in thickness. In the upper portion 

 of these deposits, volcanic tuffs or ashy materials are interstratified 

 with ordinary muddy sediment, and here and there associated with 

 thick beds of felspathic lava. These rocks form the mountains called 

 the Arans and the Arenigs ; numerous greenstones are associated 

 with them, which are intrusive, although they often run in the lines 

 of bedding for a space. " Much of the ash," says Professor Ramsay, 

 " seems to have been subaerial. Islands, like Graham's Island, may 

 have sometimes raised their craters for various periods above the 



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

 f Ibid., p. 212. X Ibid -> P- 325> 



