part 3] silueian rocks of the clun-forest district. 225 



III. Classification and Description of 

 the Strata. 



The stratigraphical succession is as follows : — 



TJi ickness 



in feet. 



OldRed Sandstone (P) 1 — 



Temeside Shales 350 



Downton-Castle Sandstone Group 110 



Chonetes Beds < T * ^ OAA 



[ Lower 300 



Rhynchonella Beds 300 



Dayia Shales ? 300 



Lower Ludlow Shales — 



1410 



It will be noticed that the grouping is similar to that adopted in 

 the Ludlow district 2 except that the A ymestry Limestone is absent 

 from the Clun -Forest area, and that the Chonetes Beds are sub- 

 divided into an upper and a lower group. It will be shown later 

 that the upper division of the Chonetes Beds is to some extent 

 comparable with the Spirif'era-elevata Shales of Ludlow. 



The thicknesses stated above are only approximate, but they are 

 interesting as a basis for comparison with other districts. 



At Mocktree Hill (see map, fig. 1, p. 222) the total thickness, 

 including the Aymestry Limestone (250 feet), is 850 feet ; around 

 Ludlow 3 the total thickness is only about 550 feet, including the 

 100 feet belonging to the Aymestry Limestone ; farther north- 

 eastwards the beds are still thinner, totalling about 180 feet in 

 South Staffordshire, as described by Mr. W. W. King & Mr. 

 W. J. Lewis. 4 



Although each of the above divisions may be described as having 

 its own characteristic lithology, there is a notable tendency to a 

 repetition of characters which is often very confusing. Moreover 

 there is, with perhaps one exception (namely, above the Downton- 

 Castle Sandstone), a complete lithological. transition from one set 

 of beds to the next. Hence it is difficult to determine satisfactory 

 divisional lines for use in mapping. This difficulty is accentuated 

 in the case of the Rhynchonella Beds, the identification of which 

 depends largely on negative evidence, namely, the absence of 

 Chonetes. 



1 See note, p. 242. 



2 G. L. Elles & I. L. Slater, Q. J. G. S. vol. lxii (1906) p. 198. 



3 Ibid. pp. 199 et seqq. 



4 Geol. Mag. dec. 5, vol. ix (1912) pp. 437 et seqq 



