Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/555

Rh regarded as unquestionably of Bala age. This is shown conclusively by the Brachiopods, and especially by Trilobites of the following forms—Sphærexochus mirus, Cheirurus juvenis, C. cancrurus, C. gelasinosus, C. octolobatus, Lichas laxatus, L. hibernicus, Illænus Bowmanni, I. Davisii, Phacops macrourus, P. apiculatus, constituting amongst the latter a characteristic assemblage of Caradoc or Bala types. It is somewhat curious that of the two commonest and most characteristic Trilobites of the Dufton shales, Calymene Blumenbachii is hardly known in the Coniston Limestone, and Trinucleus concentricus is generally absent. The latter occurs plentifully in the "Trinucleus-shales" of Prof. Hughes, a locally developed group at the summit of the Coniston Limestone in the Sedbergh district. The evidence to be derived from the Brachiopods is equally conclusive with that of the Trilobites, all the common forms being characteristic species of the Bala formation; but it is not necessary to enter into further details on this point.

It remains briefly to consider the conditions under which the Coniston Limestone series was deposited. On this matter we wish especially to indicate that there seems to be good evidence that the volcanic activity which produced the vast mass of ash, breccias, and lavas constituting the Borrowdale series, though greatly mitigated in intensity, had not entirely died out during the period of the deposition of the Coniston Limestone and Dufton Shales, but continued to operate at occasional intervals. It would seem probable that the Lake-district was not entirely submerged at the time when the Coniston series began to be laid down, but that a portion of the volcanic region remained above the level of the sea, its vents occasionally giving exit to showers of volcanic ashes or even currents of lava. If this hypothesis be established, it would follow that there was no breach of continuity between the close of the Borrowdale series and the commencement of the Coniston series, but that the two groups of rocks are intimately related, and in point of fact actually overlap one another in time. The principal grounds which at present appear to indicate the correctness of this hypothesis may be briefly stated as follows:—

(1) The intercalation in the Borrowdale series, close to its summit, of a band of fossiliferous shales containing Bala species, proves that the volcanic energy of this period still continued in force at a time when the sea was peopled with well-known Bala Brachiopods and Trilobites. The shales in question (Style-End Grassing beds) are usually separated from the Coniston Limestone proper by a mass of lava; but they are undoubtedly to be regarded as, palæontologically, a portion of the Coniston series.

(2) The presence of beds of ashes, containing numerous fossils, high up in the Dufton Shales at Swindale, near Knock, proves similarly that the volcanic eruptions of the preceding period had not entirely ceased at the time when the Dufton shales were in course of formation, and these shales, as we have seen, belong palæontologically to the Lower Coniston group, and are only a local development of the base of the Coniston Limestone itself.