About 100 feet. About 135 feet 
CARBONIFEROUS VOLCANIC ROCKS OF SOMERSET 191 
enabled to show that it was extremely probable that ( 1 ) 
the igneous rocks in the various localities occurred on one 
horizon and marked one episode ; (2) that this occurred 
at a period marked by the marine fauna of group B in the 
following table supplied by Dr. Vaughan : — 
Position of Beds re- 
ferred to the section 
Sequence 
Palseontological character- 
N. of Avon at Bris- 
tol. 
istics. 
Lower part of'j 
Great Quarry and I 
underlying Dolomi - 1 
tic Beds. J 
Higher Beds. 
' Marked by the entrance of 
Lithostrotion, Producti belong- 
- ing to the giganteus group, and 
Athyrids of the ambigua group 
JSeminula). 
Oolitic beds in-s 
the Quarry at foot 
of Gully. 
V Group C. 
Beds between 
Gully and Black- 
Rock Quarry. j 
/ Marked by the great abund- 
ance of Ortkoietes {Streptorhyn- 
chus) crenistria and Chonetes 
■< papilionacea and aff. comoides. 
In these beds Spirifer aff. 
laminosu8 reaches its maxi- 
mum. 
Upper Third of 
Black-Rock 
Quarry. 
1 Group B. 
resting immediately 
upon 
I Group xA. 
( Marked by the abundance of a 
ZapJirentid of the cylindrica 
type {Campophyllum=Zaph- 
rentis cylindrica of Edw. & 
Haime). 
J- Containing cornute Zaphren- 
( tids in great abundance. 
Lower and Middled 
Thirds of Black > 
Rock Quarry. J 
f Marked by the rarity of 
Zaphrentids and by the gradual 
increase downwards of Spirifers 
of the bisulcatus group ; and of 
the glaber type {Martinia) ; 
Lower Beds. •< Athyrids of the Royssii type 
(Cleiothyris) ; Orthids of the 
Michelini and resupinata types, 
Leptena analoga, and Pro- 
ducti of the punctatus and 
^Martini types. 
As regards the petrology of the igneous rocks, they may 
in brief be said to be all basaltic in character. The lavas 
p 
