TRAP ROCKS. 



109 



stones and shales are red, in the other they are white, or yellowish white, and black ; 

 yet notwithstanding these discrepancies in structure and in colour within so short a 

 distance, the limestones of Lilleshall and Steeraway are geologically identical, and were 

 doubtless formed during the same period and in the same sea, for they contain similar 

 marine shells and corals, and occupy precisely the same place in the series ; that is, 

 they are overlaid by and graduate upwards into those sandstones and grits which form 

 the base of the productive coal-field. 



The carboniferous limestone is the base of the system under consideration. The 

 characters and relations of the other deposits which lie beneath this coal-field will 

 be explained in a subsequent part of the work ; and I shall here merely state that the 

 carboniferous limestone has not in this locality any regular downward passage into 

 the Old Red Sandstone as in other districts. On the contrary, the Old Red terminates 

 at the southern end of the tract, and has never been found beneath the coal measures 

 or mountain limestone on the north bank of the Severn • the underlying stratified rocks 

 throughout the productive coal-field consisting of various members of the Silurian 

 System. Of these the Wenlock limestone or second formation of that system is much 

 the most conspicuous. Rising to the surface on both banks of the Severn, it is pre- 

 sented in juxtaposition to the carboniferous limestone ; so that the geologist contrasting 

 the two rocks, can at once perceive their dissimilarity both in lithological structure and 

 organic contents. 



Besides these stratified sedimentary masses, there are, however, other rocks of cry- 

 stalline structure and amorphous forms, (the trap rocks of geologists,) which in a pre- 

 vious chapter have been shown to be of volcanic origin, (p. 68.) 



Trap Rocks. — The Wrekin may be called the great centre of volcanic action in. the 

 proximity of Coal Brook Dale, but the consideration of that elevated mass is neces- 

 sarily postponed, until the nature of the strata through which it has been erupted has 

 been explained. In the mean time I simply allude to the Wrekin, because the trap 

 rocks of the coal-field on its eastern flank, might seem to be closely connected with it. 

 The volcanic agency, however, by which these contiguous masses were erupted, was in 

 activity at different periods. That which gave rise to the Wrekin, took place during 

 and after the accumulation of the Silurian System, the strata composing which were 

 thereby thrown into inclined positions, before the sedimentary matter composing the 

 carboniferous system was elaborated. At a subsequent epoch, and long after their 

 consolidation and completion, the coal measures were in their turn pierced and tra- 

 versed by other intruding masses of trap, differing in mineral characters, but erupted on 

 contiguous lines of fissure parallel to that of the Wrekin. These latter outbursts 

 specially relate to the present subject, and I may therefore say a few words concerning 

 them, although the reader will better comprehend their origin and connection, when 

 he shall have perused the chapters in which the more ancient phenomena of the same 

 class are described. 



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