﻿Vol. 6 I.] SEQUENCE IN THE BRISTOL AEEA. 209 



occur above are certainly not identical with those that are found 

 below. 



Orthotetes, which is enormously prolific throughout the lower 

 portion of the Carboniferous Limestone, becomes extremely rare 

 (and distinct in form) in the upper. 



The Producii of the upper portion belong- to the giganteid group, 

 of which group no representatives have been found in the lower part 

 at Sodbury. 



This sharp distinction of a lower system (Tour- 

 naisian) from an upper (Visean) is at Sodbury a 

 definite fact which cannot be overlooked. It is only 

 by the study of the whole series in other parts of the Bristol 1 

 area, where the transition becomes more gradual, that the com- 

 plete distinction of an upper and a lower system is seen to break 

 down. At Sodbury the occurrence of Caninia is an isolated 

 phenomenon of the Lower Visean ; farther south and west, this 

 genus is seen to be the result of direct evolution from Zaphrentis 

 in the Upper Tournaisian. 



[The general resemblance of the lithological character of thia 

 zone in the two sections is striking : — 



The poorly-fossiliferous encrinital limestones at the base (top of 

 the Lower Limestone), the band of oolite (equivalent to the Gully 

 Oolite), the shales and dolomites (equivalent to the Middle Shales), 

 and the massive limestone at the top (equivalent to the bottom of 

 the Middle Limestone) are represented in the same order in the 

 two sections, and the beds have similar textures, 



The differences are of less importance, and consist (1) in an 

 inflation of the dolomite-bands at Sodbury : therefore, in that 

 section, the Middle Shales would be more aptly termed 'the 

 Middle Dolomites ' ; and (2) in the occurrence of the highly - 

 quartzose grit-band.] 



Visean. 

 Seminula-Zone (including S x and S 2 ). 



Sj has already been discussed under the Caninia -Zone, but 

 is again included here for the reasons already given. 



Lithological character: — 

 „ J Shales, dolomites, and grits. 



A long series of massive limestones, containing thick oolitic 



beds, are succeeded by a thick series of concretionary beds 



(' mottled limestones ') with shale-partings. The mottled 



limestones exhibit the peculiar character of Cotham 



^ J Marble, and have a very similar concretionary surface ; 



2 ^. Mr. A. Strahan l has pointed out their similarity to the 



Mumbles-Head Beds. 



j The uppermost part of the zone is composed of thick shales 



1^ with several bands of grit. 



1 Mem. Greol. Suit. ' Summary of Progress Keport for 1902 ' p. 193. 



