﻿Vol. 64.] THE FOSSILI.FEROTJS SILUHIAN EOCKS OF TORTWORTH. 535 



IV. Mutual Eelations op the Rocks, a:n-d Earth-Movements 



AFFECTING THEM. 



The Tortworth Silurian rocks are affected by the post-Carboni- 

 ferous flexures which produced the Bristol Coal-Basin, and the out- 

 crop of the beds in the main regularly follows the horseshoe-shaped 

 outcrop of the Old Red Sandstone. This is particularly the case in 

 the Charfield-Green neighbourhood, where all the rocks dip regularly 

 in a south-westerly or south-south-westerly direction, and in the 

 region between Falfield and Whitfield, where the dip is in an easterly 

 or east-south-easterly direction. 



South of Whitfield the outcrop of the Silurian — still in con- 

 formity with that of the Old Red — swings round to the west. 



In the neighbourhood of Daniel's Wood and Middlemill, this 

 regularity is lost, and the geological structure becomes difficult to 

 understand. The two bands of trap and associated Llandovery, 

 which were approximately parallel one to the other and at an 

 uniform distance apart in the Charfield-Green and in the Damery- 

 Avening Green regions, now diverge : the lower band striking in an 

 almost northerly direction through Mickle Wood until it is probably 

 overlapped by the Trias, while the higher band swings round south- 

 wards and is brought by the thrust-fault running along the western 

 side of Daniel's Wood over the W^enlock Series. This fault is 

 probably to be prolonged past Middlemill to Woodford. 



The occurrence of the upper trap-band and overlying strata at 

 Middlemill is probably due to a second fault which shifted them 

 northwards from Daniel's Wood, the strata being further folded into 

 a syncline. 



The remarkable narrowness of the Old-Red-Sandstone outcrop 

 from Tortworth to Horseshoe Farm suggests the prolongation of the 

 fault along the western side of Daniel's Wood still farther south- 

 wards, so as to cut out the lower beds of the Old Red. We are, 

 however, of opinion that the variability in the thickness of the Old 

 Red Sandstone and the probable absence of the greater part, or even 

 locally of the whole, of the Ludlow Series, are to be attributed to 

 upheaval and erosion of the area in late Silurian and early Devonian 

 times.^ This would be in conformity with what apparently took 

 place in South Wales "^ and in the Mendips.^ 



V. General Remarks on the Fossils (by F. R. C. R.). 

 Llandovery. 



The separation of Lower and Upper Llandovery horizons in this 

 area is not very obvious from the palaeontological evidence. The 

 distribution of the brachiopoda is generally a guide, and the relative 



^ Prof. Lloyd Morgan makes practically the same suggestion in the Excursion- 

 Handbook, No. 17, Brit. Assoc. (Bristol) 1898, p. 12. Phillips also clearly 

 recognized that there was a non-sequence between the Tortworth Silurian and 

 the Old Eed. His words (Mem. Geol. Surv. vol. ii. pt. i, 1848, p. 197) are :— 

 ' We can admit here only the upper part of the Old Red Series ; the lower part is 

 entirely deficient ... we have ... a case of real unconformity of area between 

 the Old Eed and the Silurian strata, without local disturbance of the dips.' 



2 * Summary of Progress of the Geological Survey for 1901 ' (1902) pp. 38-40 



3 Quart. Journ. Geol. Soc. vol. Ixiii (1907) p. 234. 



