230 



ANCIENT VOLCANIC GRIT COMPARED WITH MODERN. 



stone extends from the hills between Church Preen and the Hoar Edge, to Broome, 

 Chatwall, &c., and is re-produced at intervals for some miles, extending to Cheney 

 Longville, where it has been described as composing a part of the third formation of 

 the Silurian System. These beds consist, for the most part, of slightly micaceous and 

 very fine-grained sandstone, of a dingy olive-green colour ; they contain casts of encri- 

 nites, trilobites, and mollusca, but if these fossils were excluded, the stone would be 

 said to resemble a sandy claystone of the trap family. 



These strata, however, mark the extreme verge of this class of deposits, for they are 

 surmounted by the argillo-calcareous formation of Wenlock, neither in which nor in the 

 overlying Ludlow rocks are there beds of similar structure. 



As I have detected these sandstones, not only on the sides of the Salopian intrusive rocks, 

 but also on the flanks of the Malverns and in other places where syenite and other trap 

 rocks protrude, and as I have never observed them where there are no such rocks in the 

 vicinity, I am induced to think they were formed during a period of submarine volcanic 

 action, and that the materials are ashes and scoriae given off during submarine ejec- 

 tions. 



Such deposits must, it is evident, be now forming beneath the sea where submarine 

 volcanic eruptions occur, and the mass of Graham Island, as well as the fine detritus 

 resulting from similar causes, described by Capt. Smyth and others, as rendering the 

 Mediterranean turbid, must have subsided in a manner directly analogous to that by 

 which it is conceived this grit was accumulated. 



That organic remains should occasionally be found in the ancient grit is strongly 

 supported by modern comparison, for the eruption above noticed must have entangled 

 the shells at the bottom of the sea, and many of the dead fishes noticed on the surface 

 most probably were entombed in the subsiding ashes. 



In this instance we have, therefore, a striking example of the value of studying 

 existing operations, with a view to explain ancient phenomena 1 . 



I must first refer the reader to the fifth chapter for a general explanation of this 

 subject, and to chapters 22, 23 and 25 for copious details and illustration of my views. 

 The evidences there adduced are clear, that these volcanic grits were formed during the 

 elaboration of the Lower Silurian Rocks, and were the precursors of great outbursts of 

 intrusive trap along the principal fissures of eruption. 



Period of Eruption of the Caradoc. 



If the volcanic grits are proofs of often repeated igneous action during the period 

 when the Lower Silurian strata were accumulating, a singular outlier of Upper Silu- 

 rian rock on the north-western flank of Caer Caradoc, distinctly proves that the amor- 

 phous trap of the main ridge was subsequently thrown up. Near the base of one of the 



1 See Lyell's Principles of Geology, 4th edit. vol. ii. p. 199, &c. 



