418 J. Prestwich, Esq., on the 



Lower Silurian Rocks. 

 PI. XXXV. and XXXVI., fi-. 5, 6, 7, 14, and 15. 



Caradoc Sandstone. 



This division of the great system, worked out by Mr. Murchison in the 

 border counties of England and Wales, is in the Coalbrook Dale district, but 

 imperfectly developed. Its lower member is a coarsely-grained quartzose 

 grit, which forms a belt around the trap of the Wrekin, Ercal, and Lilleshall 

 hills. The strata incline at a high angle against their flanks, but the dip 

 rapidly decreases as we recede from them. Portions of the rock disintegrate 

 rapidly, owing to its broken state and loose texture ; but the sandstone or grit 

 is frequently very pure, and occasionally so free from iron, that it has been 

 quarried for the use of the china manufactory at Coalport. Mr. Murchison 

 attributes its clear white colour and friable texture, to the action of the igneous 

 rocks which protrude through it. Between the Wrekin and the Ercal it pre- 

 sents the appearance of a gravel, composed of small angular fragments of hard 

 grit, mixed with a few larger masses, and coloured, as well as slightly cemented, 

 by the peroxide of iron. As far as my observation extended, this rock does 

 not contain any organic remains. At the south-eastern extremity of the 

 Wrekin, near the road from Leighton to Wenlock, and in a quarry by the 

 road side, at the eastern entrance of the glen, which divides the Wrekin and 

 the Ercal hills, there reposes upon this quartzose grit, a dark greenish mica- 

 ceous flag, the mica being so abundant at a pit, on the western side of the 

 lower Wrekin, that the rock disintegrates into a fine powder. It is, however, 

 generally small-grained and hard, breaking into rhomboidal masses. At the 

 second quarry between the Wrekin and Ercal hills, the flagstone is in immediate 

 contact with the basalt, which has tilted it into a nearly vertical position ; but 

 the general dip of this rock is conformable to that of the underlying grits. I 

 could discover in it no trace of fossils. The strata in the immediate vicinity of 

 the Wrekin and Ercal are so broken and shattered by the protrusion of the ig- 

 neous rocks, that it is extremely difficult to trace their continuity ; and the 

 schist is not often exposed, aUhough it may probably form a narrow belt, from 

 ten to twenty yards broad, parallel to the underlying grits. 



As we recede from the Wrekin, coarse- and fine-grained ochreous and 

 greenish sandstones, alternating with beds of light gray clay, dark gray and 

 liver-coloured shales, and impure gray limestones, make their appearance. 

 These beds have been recognized by Mr. Murchison, as belonging to the 

 Caradoc Sandstone. 



They range on the eastern side of the Wrekin, and parallel to ils major 



