Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/547

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the following communication we propose shortly to consider the various groups of strata which intervene between the great volcanic series (Borrowdale Rocks) of the Lake-district and the well-marked band of sedimentary rocks to which Prof. Sedgwick applied the name of the "Coniston Flags." In so doing we shall have occasion to note, in a general way, the physical characters and relations of the successive deposits in question; but we shall have to draw attention more especially to the organic remains which they contain and to some indications thus afforded as to their precise age and position in the geological scale.

The base of the great Silurian series of the north of England is constituted, as is well known, by the "Skiddaw Slates," a thick mass of sediments, originally in the condition of black mud, clearly proved by their fossil contents to be of the age of the "Arenig group" of Wales. Succeeding the Skiddaw Slates there occurs a great series of volcanic products, termed by Prof. Sedgwick the "green slates and porphyries," to which we have elsewhere given the name of "the Borrowdale series." These consist of ashes and breccias, alternating with ancient lavas, a portion of the series being subaerial, whilst part is of submarine formation.

Throughout the greater part of this extent, for a thickness of some thousand feet, the Borrowdale series has hitherto proved unfossiliferous; fossils, however, make their appearance in a thin band of calcareous ashes near the summit of the group (Harkness and Nicholson, Quart. Journ. Geol. Soc. vol. xxii. p. 480).

In some places, as at Style-End Grassing, between Long Sleddale and Kentmere, this band consists of brownish or bluish grey shales, and it is separated from the Coniston Limestone by a bed of trap. In other spots, as at Sunny Brow, between Ambleside and Coniston, the bed is siliceous and gritty in nature. At Millom the same fossiliferous band is recognizable, and is not only ashy in its character, but is surmounted directly by strata belonging to the Coniston Limestone series without the intervention of traps. Lastly, there are places, such as the east side of Long Sleddale and the southern declivity of Wansfell, where the same beds can be readily recognized, having the same relation to Coniston Limestone, similar to the ordinary ashes of the Borrowdale series in their ordinary character, and exhibiting no traces of fossils beyond the presence of innumera-