Cybele verrucosa, Illænus Bowmanni, and Lichas laxatus, can leave no doubt as to the correctness of the inference that these rocks belong to the Bala or Caradoc age. The presence of what appear to be unequivocal ashes which contain some of these fossils high up in the series at Swindale also deserves attention as showing that the volcanic forces which gave rise to the ashes and lavas of the Borrowdale group still maintained an intermittent activity during the deposition of the Dufton Shales. There would thus appear to have been no break of continuity between these shales and the underlying Borrowdale rocks—a conclusion which is further borne out by the substantial identity between the fossils of the Dufton Shales and those of the Style-End Grassing beds.
The annexed section (fig. 1) exhibits the stratigraphical relations of the Dufton Shales in Swindale Beck, where, as before stated, they are very well exposed.
Fig. 1.—Sketch Section of the Strata in Swindale Beck, near Knock.
(Length rather more than half a mile.)
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a. Ashes belonging to the Borrowdale series c. Coniston Limestone. e. Knock beds (green and purple slates).
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b. Dufton Shales. d. Graptolitic mudstones. |
f. Black flags with Monograptus colonus, Barr., probably Coniston Flags.
2. Coniston Limestone.
The "Coniston Limestone," notwithstanding its comparatively small vertical extent, has long occupied the position of being the best-defined and most universally recognized of all the divisions of the Lower Silurian series of the north of England—a position which it owes to its easily recognized lithological characters, and to the number of organic remains which it has yielded. It is unnecessary here to recapitulate the geographical range of the Coniston Limestone. Its main line of outcrop crosses the Lake-district in a direction from S.W. to X.E., running from Millom on the one hand to Snap Wells on the other. It is more or less developed in Ravenstonedale, Dentdale. the Sedbergh valley, near Ingleton, in Ribblesdale, at Ireleth, at High Haulme. in Furness. and at various points in the Lower Silurians which lie to the south-west of the Cross-Fell range.
Lithologically the term "Coniston Limestone" is somewhat misleading, as it is never wholly calcareous in its composition, and the calcareous element is occasionally almost wanting. In its most typical form, as seen in its range between Long Sleddale and Broughton Mills, in Furness, the Coniston Limestone consists of
