[262] Messrs. Harker and Marr, Quart. Journ. Geol. Soc. vol. xlvii. (1891), p. 310.
The absence of interstratifications of ordinary non-volcanic sediment in the Borrowdale group might conceivably arise from the eruptions following each other so continuously on the sea-floor, and at so great a distance from land that no deposition of sand or mud from the outside could sensibly affect the accumulation of volcanic material. Certainly some miles to the east at the Cross Fell inlier, 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 confines 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 my mind of subaqueous than of subærial 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 the 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 unfrequent, and certainly play a much more prominent part here than they do in the Welsh 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 uprise of the post-Silurian granitic magma rather than with the history of the volcanic group.[263] 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.
[263] For a description of the dykes around the Shap granite see the paper by Messrs. Harker and Marr, Quart. Journ. Geol. Soc. vol. xlvii. (1891), p. 285.
vii. UPPER SILURIAN (?) VOLCANOES OF GLOUCESTERSHIRE
A remarkable group of igneous materials has long been known to rise 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 Wernerian sense by Weaver.[264] Murchison regarded them as intrusive sheets;[265] Phillips looked on them as partly intrusive and partly interstratified.[266] They consist largely of coarsely-amygdaloidal basalts, some of which have been microscopically examined.[267] But their field-relations as well as their petrography have not yet been adequately determined. They are represented on the Geological Survey Map as forming a number of parallel bands in strata classed as Upper Llandovery. If, as seems probable, some of them are really interstratified, they form the youngest group of Silurian volcanic rocks in England, Scotland, or Wales.
[264] Trans. Geol. Soc. 2nd ser. vol. i. (1819), pp. 324-334.
[265] Silurian System (1839), p. 457.
[266] Mem. Geol. Surv. vol. ii. part i. (1848), p. 194.
[267] "Geology of East Somerset," etc., in Mem. Geol. Surv. (1876), p. 210; descriptions by Mr. F. Rutley.