SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 11 
in many cases are considered by Prof. Boulton to have been picked up 
in the form of calcareous mud, and squeezed into the spaces between 
spheroidal masses of basalt. Other sections of volcanic rocks occur at 
Woodspring, three miles to the north of Spring Cove, and at Goblin 
Combe, near Wrington ; but at each of these sections the lava plays a 
subordinate part, the principal igneous rocks being calcareous tuffs, 
which at Woodspring have a thickness of upwards of 100 feet. It is 
noteworthy that at Woodspring and Goblin Combe the igneous rocks 
are not quite contemporaneous with those at Spring Cove, occurring as 
they do in the uppermost part of the Zaphrentis beds, i.e., some 350 
feet below those at Spring Cove. 
At the top of the Syringotliyris Zone the Tournaisian or Lower 
Carboniferous Limestone ends, the succeeding Seminula Beds belonging 
to the Upper Carboniferous Limestone or Visean. These rocks, con- 
sisting as they do in the main of massive limestone, are frequently 
quarried, and hence are excellently exposed in many parts of the Bristol 
district, as in the Avon and Wickwar quarries and the Cheddar Gorge. 
While certain fossils, especially Lithostrotion among the corals and 
Semimda among the brachiopods, occur in great abundance, the Seminula, 
Beds are characterised by a relatively impoverished fauna, showing 
little variety as compared with that in the overlying and underlying 
strata. The beds are however decidedly variable lithologically, and 
include a thick mass of wLite oolitic limestone, the Seminula oolite, and 
in the upper part a series of peculiar concretionary beds well seen in the 
cutting and quarry at Chipping Sodbury. 
The uppermost division of the Carboniferous Limestone, the 
Dihunophyllnm Zone, is again highly fossiliferous, being specially 
characterised by the multitude of corals of the genera Lithostrotion, 
Lonsdaleia, Dihunophylhim, and Alveolites. The great majority of the 
Bristol corals shown in museums are derived from these beds. Though 
well exposed in the Avon section, the Dibunophyllurri Beds are not so 
often seen as are the Semimda and Zaphrentis Beds. They are how- 
ever well exposed, and very fossiliferous in the quarries at Flax Bourton 
and Wrington. They show much lithological variation, and include 
beds of grit, shale, and conglomerate. They were formerly known as 
the “ Upper Limestone Shale,” a term which, like the corresponding 
“ Lower Limestone Shale,” should now be regarded as of merely litho- 
logical significance, and of value only when the denudation features of 
the district are under consideration. 
Upper Carboniferous Series.^ 
The limestones of the Lower Carboniferous rocks are succeeded by 
a mass of very hard red grit, with layers of shale and conglomerate, 
^ For the Coal Measures see H. B. Woodward “ Geology of East Somerset 
and the Bristol Coaltields”, ‘Mem. Geol. Surv. of England and Wales’, 1876 ; 
J. McMurtrie “ The Coal-fields of Somersetshire”, ‘ Proc. Somerset Arch, and 
Nat. Hist. Soc.’, XIII., pt. 2, p. 119; other papers in the ‘Proc. Bath Nat. 
Hist, and Antiq. field club’, vol. 1 (3), p. 127, ibid., vol 2, page 454, etc. ; 
also J. Anstie, ‘ The Coalfields of Gloucestershire and Somersetshire ’, 1873. 
