PERIODS OF ERUPTION AND REPOSE. 



235 



the bedded structure becomes still more decided, and the rock is a thin, flaglike claystone, dipping 

 eighty degrees north-west, in highly inclined beds. Towards the southern end of these great 

 quarries, the flaglike rocks are traversed by several dykes running from north-west to south-east, 

 issuing as it were from the body of the hill. The broadest of these dykes is about ten feet wide. 

 They are also felspar rocks, of dingy green and greenish grey colours, of a rough fracture, slightly 

 resembling certain trachytes, and are in part coloured purple by the oxide of iron. The inclosing 

 or contiguous masses of rock are much harder than the dykes themselves, presenting the appearance 

 of having been altered by the injection of the latter. Similar masses run in devious directions 

 through the trap of the Caer Caradoc. As both the mass of the hill and the dykes, are rocks of 

 igneous origin^ we have only to suppose that the latter are posterior injections filling up chinks and 

 cracks, and cutting through the body of trappsean or volcanic dejections 1 . The trap of Lilleshall 

 Hill has risen through thin-bedded sandstones, which are apparently of the same age as those of 

 the Caradoc. This sandstone is seen in several parts of the village of Lilleshall, particularly near 

 the church, where it is highly inclined, and unconformable to the carboniferous limestone. Some 

 beds belonging to the bottom of this limestone rise up upon the north-western face of the hill from 

 beneath the adjoining large mass of that rock, (see p. 107.) Although the relations of these beds 

 are not clearly displayed, they have certainly been affected by the trap, since in the prolongation of 

 the axis of the hill to the north-east, the limestone is thrown off to the north-east and north-west, 

 the axis of elevation being directly upon the prolongation of the line of eruption of Lilleshall Hill, 

 and also in the continuation of the great Lightmoor fault by which the whole coal-field and the 

 underlying Silurian rocks have been so powerfully affected. (See PI. 29. f. 15.) 



Coupling the preceding observations with what has been said concerning the trap 

 rocks and dislocations of Coalbrook Dale and the Clee Hills, it may be affirmed, that 

 this district in Shropshire furnishes proofs of the alternate play and repose of volcanic 

 action during very long periods. These evidences demonstrate, 1st. That volcanic grits 

 were formed during the deposition of the Lower Silurian strata ; 2nd. That the Upper 

 Silurian rocks and Old Red Sandstone were accumulated tranquilly without a trace of 

 contemporaneous eruptions ; 3rd. That after their consolidation, the last mentioned 

 deposits were dismembered and set upon their edges by vast outbursts of intrusive trap ; 

 4th. That the carboniferous system was deposited after the older strata had been up- 

 heaved ; 5th. That subsequent dislocations, including some of the most violent with 

 which we are acquainted, took place after the accumulation of the coal measures, and 

 Lower New Red Sandstone. 



1 If an illustration of this be required, the reader is referred to the writers who have described Vesuvius. 

 On the principal sides of Monte Somma, the ancient and extinct crater of Vesuvius, are numerous dykes of 

 lava penetrating in vertical and oblique directions, the former coulees and dejections of volcanic matter. These 

 facts were first pointed out to geologists by Sir James Hall. They have since been dwelt upon by Necker de 

 Saussure and other writers, and more recently by Mr. Lyell. I may here remind my readers, who are not 

 geologists, that the difference between the volcanic action at Monte Somma, and in Shropshire, consists in 

 this, that the eruptions in the former case are sub-aerial, in the latter they were sub-marine. 



