238 
THE SILURIAN VOLCANOES 
BOOK IV 
east at the Cross Fell ialier, as already mentioned, there is evidence of the 
alternation of tuffs with the sandy and muddy sediment of the sea-bottom. 
Here, at the outer coniines of the volcanic district, the ejected materials 
evidently fell on the sea-floor, mingled there with ordinary sediment, and 
enclosed the same organic remains. The well-defined stratification of many 
of the fine tuffs is rather suggestive to ray mind of subaqueous than of 
suliaerial accumulation. At the same time, there seems no reason why, 
here and there at least, the volcanic cones should not have risen above 
the water, though their materials would be washed down and spread out by 
the waves. 
One of the most marked points of contrast between the Cumbrian and 
tlie Welsh volcanic districts is to be found in the great paucity of sills in 
the former region. A few sheets of diorite and diabase have been mapped, 
especially in the lower parts of the volcanic group and in the underlying 
Skiddaw Slates. On the other hand, dykes are in some parts of the district 
not itnfrequent, and certainly play a much more prominent part here than 
they do in tlie W^elsh volcanic districts. The majority of them consist of 
felsites, quartz-porphyries, diorites, and mica-traps. But there is reason to 
suspect that where they are crowded together near the granite, as around 
Shap Fells, they ought to be connected with the uquise of the post-Silurian 
granitic magma rather than with the history of the volcanic group.^ If this 
series of dykes be eliminated, there remain comparatively few that can with 
any confidence be associated with the eruption of the Borrowdale rocks. 
vii. UPrER SILURIAN (?) VOLCANOES OF GLOUCESTERSHIRE 
A remarkable group of igneous materials has long been known to I’ise 
among the Silurian rocks of the Tortworth district at the north end of the 
Bristol coal-field. They were believed to be aqueous deposits in the W'^ernerian 
sense by Weaver.- Murchison regarded them as intrusive sheets ; ® Phillips 
looked on them as partly intrusive and partly iuterstratified.'* They consist 
largely of coarsely-amygdaloidal basalts, some, of which have been micro- 
scopically examined.^’ But their field-relations as well as their petrography 
liave not }'et been adequately determined. They are represented on the 
Geological Survey Map as forming a number of parallel bands in strata 
classed as Upper Llandoveiy. If, as seems pi'obable, some of them are 
really interstratified, they form the youngest group of Silurian volcanic 
rocks in England, Scotland, or Wales. 
' For a description of the dykes around the Shap granite see the paper by Messrs. Harker 
and Marr, Quart. Juum. Ceol. Soc. vol. .xlvu. (1891), p. 28o. 
^ Trims. Gcol. Soc. 2nd ser. vol. i. (1819), pp. 324-334. 
” Silurian System {1SS9), p. 457. * Mem. Oeol. Sun. vol. ii. parti. (1848), p. 194. 
'■ “Geology of East Somerset,” etc., in Mem. Geol. Surv. (1876), p. 210 : descriirtions by Mr. 
F, Entley. 
