Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/548

462 ble cavities filled, or partially filled, with rusty peroxide of iron, apparently due to the decomposition of organic remains.

It is not our purpose to discuss here the position and age of these "Style-End Grassing beds," as they may be called. It may, however, be noted that the fossils which they have yielded are Bala types, such as Calymene Blumenbachii, Brongn., Orthis vespertilio, Sow., and Petraia æquisulcata, M'Coy. This is of interest as indicating that the volcanic activity of which the Lake-district became the theatre subsequent to the deposition of the Skiddaw Slates, continued to prevail, at any rate, up to the later portion of the Bala period.

Resting, apparently with perfect conformity, upon the Borrowdale rocks a series of deposits occur which we wish more especially to discuss here.

These deposits may be grouped in the following ascending order:—

The "Dufton Shales" are but locally distributed, though they constitute a well-marked group of muddy sediments underlying the Coniston Limestone proper and its associated shales. They do not appear in any recognizable form beneath the main hue of the Coniston Limestone in the Lake-district itself. They do not seem to occur in the Sedbergh district; nor have they been recognized in Ravenstonedale or Ribblesdale; but they are very well developed in the Silurian area which includes the Cross-Fell range.

Here they are seen in four principal exposures, owing to the folding and faulting of the strata, viz.:—Swindale Beck, near Knock; Pusgill and Dufton-Town dykes, near Dufton; Harthwaite Gill, near Keisley; and at the Smelt Mill, near Hilton.

The thickness of the Dufton Shales in these localities probably exceeds 300 feet. They consist of dark flaggy shales with a rough cleavage, sometimes (as in Swindale) having brownish or greenish ashy beds intercalated among them. These shales are readily distinguished from those associated with the Coniston Limestone, being usually darker in colour, and more flaggy, and having a less perfect cleavage. Near their base they have two bands of nodular limestones in them; these are well seen in Swindale Beck.

The Dufton Shales are richly fossiliferous throughout, this being the case with the ashy beds before referred to, as well as others of the series.

The fossils are, for the most part, characteristic Bala types; and the entire deposit may be regarded as forming, palaeontologically, the base of the Coniston Limestone.

The following is a list of the principal fossils which have been collected in these beds:—