Vol. 67.] THE CARBONIFEROUS SUCCESSION IN GOWER. 539 



beds that differ lithologically from the ' black lias ' at the same 

 horizon in being sandy instead of argillaceous. This lateral passage 

 is observable practically due north of the change from pure limestone 

 to ' black lias' in Gower. In other words, sand was being deposited 

 in the north contemporaneously with mud at a considerable distance 

 to the south ; while at a short distance to the west of both places 

 the water was clear. Such conditions are most readily explained 

 on the supposition that the region lay at the western margin of 

 the sediment brought into the Avonian sea by a southward-flowing 

 river. 



(viii) The deposition of ' black lias ' in South-Western Gower 

 above D 2 , while it persisted in the Eastern District, points to an 

 extension of the muddy waters described in the preceding 

 paragraph. 



(ix) This widespread and progressive incoming of argillaceous 

 rocks and sponge-cherts towards the end of D probably foreshadowed 

 the marked advent of shales and cherts, spicular or radiolarian, at 

 the commencement of P. The P-shales, therefore, may, also, have 

 derived much of their material in the area here described from a 

 southward-flowing river. With this view their general character 

 and the lateral passage a northwards of some part of them, or of 

 the overlying shales, into the comparatively-coarse basal Millstone 

 Grit of the North Crop are in full agreement. 



Conclusions. 



(1) In the facies of Z and the lower half of the Laminosa Dolo- 

 mites the Eastern District resembles the North- Western, and both 

 differ from South-Western Gower ; but in that of the rest of C 

 there is no marked difference between the districts : while, in the 

 development shown by C a , S, and D 1? the Eastern District links 

 itself with the South-Western and both differ from North- Western 

 Gower. 



(2) These resemblances and differences are probably attributable 

 to the fact that the depth of the Avonian sea and the influence 

 of the Avonian coast differed in the several districts ; for similar 

 differences are observable in Pembrokeshire also, in different outcrops 

 bearing the same relations to the probable position of the Avonian 

 coast as do the parallel districts of Gower. 



(3) The evidence afforded by the thicknesses of the zones and 

 subzones in the different districts (Table I, facing p. 505) leads to 

 the same grouping as the evidence summarized in (1). Thus: — 



Zone Z and the Laminosa Dolomites together are at least 50 per cent, 

 thicker in the South-Western than in the Eastern and North- Western 

 Districts ; 



The Caninia Oolite and C 2 , though variable, show no marked increase 

 in any direction ; 



Zone S is 50 per cent, thicker in the South-Western and Eastern than 

 in the North- Western District ; and 



Zone D is thicker, probably in even greater ratio, in the South-Western 

 and Eastern than in the North-Western District. 



1 A. Strahan. Swansea Memoir, p. 29. 



Q.J.G.S. No. 268. 2p 



