Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/556

470 (3) The presence, near the summit of the Coniston Limestone itself, at Shap Wells, of a calcareous breccia containing numerous fragments of ash proves that an eruption took place towards the close of the period during which the Coniston Limestone was deposited. Nor does the site of this eruption appear to have been very far removed from Shap Wells itself; for the microscopic investigation of this breccia indicates that some of the older and previously formed beds of the Coniston Limestones were broken up by this outburst, and were thrown over the sea-bottom along with innumerable fragments of ash, the whole being subsequently cemented together by calcareous ooze to constitute the singular stratum in question.

(4) The presence in the Coniston Limestone in the Sedbergh district, as shown by Prof. Hughes, of interbedded felstones, proves conclusively the occurrence of volcanic eruptions contemporaneous with the deposition of the limestone, in this area at any rate, if not elsewhere.

(5) By the supposition we have brought forward a satisfactory explanation is obtained of a certain amount of apparent discordance between the Coniston Limestone and the underlying volcanic Borrowdale rocks. This discordance, so far as it exists, might be set down as due to a want of conformity; but we do not think that this is its true explanation.

If the views we entertain be correct, it is rather due to the fact that the Coniston Limestone was deposited round the shores of the volcanic nucleus of the Lake-district very much after the fashion that a modern limestone might have been in process of formation for thousands of years off the coasts of Sicily. In such a case beds of limestone would wrap round sheets of lava so as to be apparently transgressive thereon, or might be interstratified with strata of tuff, of ash, or of volcanic rocks. Any seeming discordance between the calcareous series and the volcanic products would be due, not to absolute unconformability, indicating a lapse of time, but simply to the difference in the method by which the two groups were formed. In spite of any apparent discordance, both groups would belong to the same period, and in part they would be actually contemporaneous.

If these inferences be confirmed by further researches, it will follow that the Borrowdale rocks must be regarded as being of Lower-Bala age. As the Skiddaw slates are unquestionably Arenig, and the Coniston Limestone equally unquestionably Bala, the only other view which could be taken as to the Borrowdale series would be to refer them to the Llandeilo. Apart, however, from the considerations just mentioned, this view is rendered unlikely by the fact that the great series of Llandeilo rocks developed in the south of Scotland appears to be wholly free from intermixture with contemporaneous igneous matter. We can hardly suppose that such could possibly have been the case, if the Llandeilo strata of the southern uplands of Scotland had been in process of formation at a time when the closely adjoining region of the Lake-district was the