VOLCANIC GRITS OR CONTEMPORANEOUS TRAP. 



269 



seven miles long and is flanked on both sides by smaller parallel ridges of hard trap, 

 alternating with valleys formed in the shale and sandstone of the Lower Silurian rocks ; 

 the trap occupying the crests, and the softer parts of the sedimentary deposits the water- 

 worn depressions 1 . 



In the low country which surrounds this tract on three sides, the Stiper Stones 

 forming the fourth, are the villages of Minsterly, Worthen, Chirbury, Church Stoke, 

 Snead, and Lydham. Although this circumjacent country is composed essentially of 

 Silurian rocks, it is succeeded at a very short distance to the north by the coal-field of 

 Pontesbury. (See pp. 81 et seq.) On the west the depression, in which is Morton Pool, 

 is occupied by the lower part of the Upper Silurian rocks, which rise into the Long 

 Mountain, 1330 feet above the sea. (PI. 32, figs. 1, 2 and 3.) The map and annexed 

 sections must be consulted to render intelligible the true nature of this district, since 

 any descriptions would fail sufficiently to explain its complicated structure, 



This tract, like others hereafter to be noticed, contains two classes of trap or sub- 

 marine volcanic rock, i. e. bedded and imbedded, each of which is divisible into several 

 varieties. The bedded trap rocks frequently alternate conformably with sandstones and 

 flags containing organic remains ; whilst the amorphous and unbedded trap rises abruptly, 

 cutting through and generally dislocating the strata. In some instances, however, these 

 two classes seem to pass into each other. (See p. 277.) 



" Volcanic Grits." 



The stratified trap rocks occur at intervals on both flanks of the Corndon or principal eruptive 

 chain, alternating with regular sedimentary deposits. We proceed therefore to point out first a 

 few cases of such alternations, as they are older than the intruded trap. In traversing the various 

 parallel ridges between Wotherton and Marrington Dingle, till we reach the flanks of the chief 

 mountain, the following succession of rocks is met with, (PL 32. f. 1.) 



1st. Beds of schist and sandstone (Chirbury and west side of Marrington Dingle). 2nd. Thick-bedded, mottled, felspathic 

 sandstones, dipping 15° beneath No. 1, and largely quarried along the eastern slopes of that gorge (Whittery quarries). The 

 upper beds, from one to two feet thick, are distinctly quartzose and felspathic, sometimes, though very rarely, containing 

 casts of shells, and are separated by courses of dark-coloured shale. Below these are beds, from three to four feet thick, of 

 mottled freestone, consisting of a light greenish grey, granular felspar, mixed with some sand, occasionally coloured dark- 

 green by chlorite, and entangling angular fragments of greywacke schist as well as porphyritic greenstone. 3rd. The 

 excellent building material, No. 2., passes down into a concretionary felspar rock, sometimes porphyritic. 



This view of one of the Whittery quarries conveys a clear idea of the arrangement of such vol- 



1 This outline of high ridges and deep furrows necessarily impedes the passage across the district, and no 

 effort was made before the year 1834 to open it out to the public by the obvious method of constructing a road 

 from north-north- east to south-south-west in one of the longitudinal valleys. This road, which was not com- 

 menced till nearly four years after my first visit, is now completed, and is already noticed in a beautiful quarter 

 sheet of the Ordnance Survey just published. Taking advantage of the longitudinal depressions, this road, 

 ascending from Minsterly by Hope Mill, passes in the hollow between Oakage and the Nick Knolls (see 

 PL 32. f. 1.) to the Grit Mines, whence it descends by the east and south of the village of Hyssington to the 

 vale of Bishop's Castle near Church Stoke. A direct communication is thus opened between Shrewsbury, 

 Montgomery, and Newtown, 



2 L 



