VOLCANIC GRIT OF THE CARADOC. 



229 



formation, whilst the mass of Hopesay Common on the east is made up of the red and 

 purple beds of the Caradoc sandstone. 



The prolongation of the chief axis of the Caradoc System is, however, to be traced through the 

 culminating point of Sibdon Hill to the village of Aston by occasional slight protrusions of highly 

 indurated and broken beds of red sandstone, with very rarely a point of felspar rock. This axis 

 finally terminates in the hill of Corton, where the Caradoc sandstone and grit have been described 

 as tilted off on either side of a line of elevation. In a recent excavation for the foundation of a 

 house on this hill, the dislocated strata were found to be based upon a rock of the same ambiguous 

 character which marks the anticlinal in Sibdon and Aston Hills, a dark red, hard, felspathic rock, 

 with a quartzose fracture, slightly tinged in parts with chlorite. 



On the south-eastern face of Sibdon Hill and a little west of Sibdon House, there is, however, an 

 outburst of unequivocal trap, which rises to the surface in three dykes more or less parallel, trending 

 towards Wartle Knoll. Two of these, consisting of decomposing concretionary greenstone, are of 

 slight thickness, with vertical and contorted beds of shale on their sides : the chief or central mass 

 rises in an arch through the strata, which from its hardness and compactness was very valuable as 

 a roadstone in this district, but was soon quarried out to a depth rendering further extraction 

 too expensive. This rock is essentially dark-coloured, felspathic, and concretionary : the white 

 felspar separating from the hornblendic base into small concretions, gives to the whole a spotted 

 aspect. Another variety is so compact a greenstone that it may almost be termed basalt, and in it 

 are minute concretions of quartz. 



" Volcanic Grit.''' 



Besides the intrusive trap and masses of altered rock, the Wrekin and Caer Caradoc 

 throw off at a little distance from their flanks, bedded sandstones and grits, which 

 although they sometimes contain organic remains, are in composition so analogous to 

 the trap itself, that I designate them "volcanic grit." Mr. A. Aikin has alluded to 

 this rock, as a micaceous sandstone nearly allied to greenstone, Geol. Trans, vol. i. 

 Old Series, p. 212. There is a good section of these beds near the southern termina- 

 tion of the Wrekin. The stone is here of a dark green colour, and consists of the same 

 materials which constitute greenstone and syenite, with a few fine scales of mica. The 

 beds dip away from the trap at a high angle. 



Again, on the eastern flank of the Caradoc, the same rock occurs in highly inclined 

 strata, also hanging off from the intrusive ridge. It is well displayed at a spot about 

 two hundred yards E.N.E. of the base of Little Caradoc, and is made up of grains of 

 green earth and felspar, with some scales of mica, &c. Hand specimens of this rock 

 would unquestionably be mistaken for trap. In other situations along the same flank 

 of that ridge, the sandstone contains much decomposed felspar. 



Somewhat further removed from the Caradoc, is a line of shelly sandstone, also 

 ranging parallel to the main ridge and dipping away from it, in which the same com- 

 position is more or less perceptible, the aspect of the rock being on the whole entirely 

 dissimilar to any sandstone of the secondary series hitherto described. This sand- 



2 F 



