With the acid intrusions may possibly be associated some of the other masses of granophyre, microgranite and granite (felsite, felstone, quartz-felsite, syenitic granite, quartz-syenite, elvanite), which have long attracted attention in this region. The largest of these intrusions is the tract of granite which stretches from Eskdale down to near the sea-coast as a belt about eleven miles long and from one to three miles broad. Another large mass is the granophyre or "syenite" of Ennerdale. Numerous other intrusions of smaller dimensions have been mapped.
To what extent any of these eruptive masses were associated with the volcanic phenomena remains still to be worked out. There seems to be little doubt that a number of them must belong to a much later period. Mr. Harker has expressed his belief that the intrusion of some of these igneous rocks was intimately associated with the post-Silurian terrestrial movements of which cleavage is one of the memorials.[258] The Skiddaw granite, though it does not touch any part of the volcanic group, but is confined to the underlying Skiddaw Slates, was erupted after the cleavage of the district, which affects the volcanic as well as the sedimentary series. In other instances also, as in that of Carrock Fell, the intrusion seems to have been later than the disturbances of the crust.[259] The amount of metamorphism around some of the bosses of granite is considerable. That of the Skiddaw region has been well described by J. C. Ward,[260] while that of the volcanic group by the Shap granite has been carefully worked out by Mr. Harker and Mr. Marr.[261]
[258] Quart. Journ. Geol. Soc. vol. li. (1895), p. 144.
[259] Op. cit. p. 126.
[260] "Geology of Northern Part of the English Lake District," Mem. Geol. Surv. 1876, chap. iii. The metamorphism around the diorites and dolerites, and the granophyres and felsites, is described in the same chapter.
[261] Quart. Journ. Geol. Soc. xlvii. (1891) p. 266, xlix. (1893) p. 359.
The Shap granite comes through the very highest member of the volcanic series, and even alters the Upper Silurian strata. It must thus be of much younger date than the volcanic history of the Lake District. It presents some features in common with the granite bosses of the south of Scotland. Like these, it is later than Upper Silurian and older than Lower Carboniferous or Upper Old Red Sandstone time. Its protrusion may thus have been coeval with the great volcanic eruptions of the period of the Lower Old Red Sandstone. It will accordingly be again referred to in a later chapter.
It must be confessed that none of the large bosses of massive rocks, whether diabases, gabbros, felsites, granophyres, or granites, appear to afford any satisfactory proof of the position of the vents which supplied the lavas and tuffs of the Lake District. Nor can such a decided accumulation of the volcanic materials in certain directions be established as to indicate the quarters where the centres of eruption should be sought. On the contrary, the confused commingling of materials, and the comparative shortness of the outcrop of the several sheets which have been traced, rather suggest that if any one great central volcano existed, its site must lie outside of the present volcanic district, or more probably, that many scattered vents threw out their lavas and ashes over no very wide area, but near enough to each other to allow their ejected materials to meet and mingle. The scene may have been rather of the type of the Phlegræan fields than of Etna and Vesuvius. If this surmise be true, we may expect yet to recognize little necks scattered over the volcanic district and marking the positions of some of these vanished cones.
What appears to have been one of these small vents stands near Grange at the mouth of Borrowdale, where I came upon it in 1890. In the little Comb Beck, the Skiddaw Slates are pierced by a mass of extremely coarse agglomerate, forming a rudely-circular boss. The slates are greatly disturbed along the edges of the boss, so much so, indeed, that it is in some places difficult to draw a line between them and the material of the agglomerate. That material is made up of angular blocks, varying in size up to three feet long, stuck in every position and angle in an intensely-indurated matrix formed apparently of comminuted debris like the stones. The blocks consist of a finely-stratified shale, which is now hardened into a kind of hornstone, with some felsitic fragments. I could see no slags or bombs of any kind. There is no trace of cleavage among the blocks, nor is the matrix itself sensibly cleaved. I believe this to be a small volcanic neck and not a "crush-conglomerate." It has been blown through the Skiddaw Slates, and is now filled up with the debris of these slates. Its formation seems to have taken place before the cleavage of the strata, and its firm position and great induration enabled it to resist the cleavage which has so powerfully affected the slates and many members of the volcanic group.
It was the opinion of my predecessor, Sir Andrew Ramsay, and likewise of Mr. Ward, that the Cumbrian volcanic action was mainly subærial. This opinion was founded chiefly on the fact that, save at the bottom and top of the series, there is no evidence of any interstratified sediment of non-volcanic kind. The absence of such interstratification may undoubtedly furnish a presumption in favour of this view, but, of course, it is by no means a proof. Better evidence is furnished by the unconformability already mentioned between the Coniston Limestone and the lavas on which it lies. Besides angular pieces of lava, probably derived from direct volcanic explosion, this limestone contains fragments of amygdaloidal andesite, and also rolled crystals of striated felspar.[262] These ingredients seem to indicate that some part of the volcanic group was above water when the Coniston Limestone was deposited.