256 



CAMBRIAN ROCKS IN SHROPSHIRE. 



by him into three principal groups. In this work I shall notice the highest of these 

 groups only, particularly where it is contiguous to the Silurian rocks. 



In Shropshire there is no passage from the Cambrian to the Silurian strata, the 

 sequence having been interrupted by the outburst of volcanic matter which formed the 

 Caradoc and Wrekin Hills, as expressed in the sections, PL 31. figs. 3 and 4. This 

 eruption, which, as was previously stated, destroyed or obscured all traces of the Llan- 

 deilo flags, has also operated in excluding certain transition beds which elsewhere 

 unite the Upper Cambrian with the Lower Silurian Rocks. In this district, in short, 

 there is a great hiatus, for there are natural sections which prove, that the Cambrian 

 group described in this chapter has been thrown into highly inclined positions, before 

 the deposition of certain beds of the Silurian System. (PI. 32. f. 1.) Not so, however, 

 on the eastern slopes of the Berwyn Mountains, nor in Caermarthenshire ; where in 

 place of an abrupt collocation as in Shropshire, there are transitions downwards from 

 the Llandeilo flags, through black schists into the older slaty rocks under consideration. 

 (PL 32. f. 9. and PL 34. f. 3 and 8.) 



In those districts it is obviously impracticable to draw a precise boundary line be- 

 tween the inferior beds of the Silurian and the uppermost strata of the Cambrian Sy- 

 stem, either by zoological, lithological, or stratigraphical distinctions. As a striking 

 example of this I may state, that the Bala or Berwyn limestone, placed by Sedgwick in 

 the Upper Cambrian group, contains several species of shells, figured in this work as 

 occurring (some of them very abundantly) in the Lower Silurian Rocks 1 . 



In the meantime, returning to the consideration of the country under review, I shall 

 now give a detailed account of the Cambrian strata with the associated trap rocks, as 

 they appear in the Longmynd and other hilly ranges in Shropshire. 



Longmynd, &c. PL 31. f. 3 and 4. and PL 32. f. 1. The Longmynd and contiguous 

 ridges on the west and north-west, comprising the hills of Ratlinghope, Pulverbatch, 

 and Linley, varying in height from 1000 to 1600 feet, consist of a vast succession of 

 strata of hard sandstone, grit, schist, and imperfectly formed slates, which are piled up in 

 mural forms, the beds being either vertical or in very highly inclined positions. The 

 strata constituting these hills have a direction from N.N.E. to S.S.W., and the various 

 ridges are of very unequal lengths. One of the longest is that which is prolonged from 

 the Linley and Pulverbatch Hills into the Lyth Hill, and finally into Haughmond Hill, 

 four miles north-east of Shrewsbury. These Cambrian rocks are flanked on the east and 

 south-east, and south, by the Silurian System and the trap rocks before described. On 

 the west, they are separated from the Lower Silurian Rocks of the mining district of 

 Shelve by a lofty ridge of quartz rock, called the Stiper Stones ; and on the north, they are 

 overlaid by the coal measures and New Red Sandstone of the Vale of Shrewsbury, as al- 

 ready explained in the sixth chapter. 



i The shells which I have collected in the Berwyn Mountain and at Bala, (Upper Cambrian of Sedgwick,) 

 and which are common in the Lower Silurian Rocks, will be enumerated in the twenty-fourth chapter. 



