558 SILURIAN YOLCANIC ROCKS. [Ch. XXXIL 



Trap of the Old Red Sandstone period. — By referring to tlie section 

 explanatory of the structure of Forfarshire, already given (p. 48), the 

 reader \Yill perceive that beds of conglomerate, No. 3, occur in the middle 

 of the Old Red sandstone system, 1, 2, 3, 4. The pebbles in these conglom 

 erates are sometimes composed of granitic and quartzose rocks, some 

 times exclusively of different varieties of trap, which, although pur- 

 posely omitted in the section referred to, are often found either intruding 

 themselves in amorphous masses and dikes into the old fossiliferous tile- 

 stones, No. 4, or alternating with them in conformable beds. All the 

 different divisions of the red sandstone, 1, 2, 3, 4, are occasionally inter- 

 sected by dikes, but they are very rare in Nos. 1 and 2, the upper mem- 

 bers of the group consisting of red shale and red sandstone. These 

 phenomena, which occur at the foot of the Grampians, are repeated in 

 the Sidlaw Hills ; and it appears that in this part of Scotland, volcanic 

 eruptions were most frequent in the earlier part of the Old Red sand- 

 stone period. 



The trap rocks alluded to consist chiefly of felspathic porphyry and 

 amygdaloid, the kernels of the latter being sometimes calcareous, often 

 chalcedonic, and forming beautiful agates. We meet also with clay stone, 

 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 green- 

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

 the axis of this chain of hills (see section, p. 48), 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. 



Silurian period. — It appears from the investigations of Sir R.. Mur- 

 chison 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 scoriae then ejected gave rise to a peculiar kind 

 of tufaceous sandstone or grit, dissimilar to the other rocks of the Silu- 

 rian 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 moUusca. 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 sedimentary strata of the 

 lower Silurian system. This trap consists of slaty porphyry and granu- 

 lar felspar rock, the beds being travei-sed 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 va- 



* Murcbison, Silurian System, <fec. p. 230. f Ibid. p. 272. 



