hard grey or bluish grey, cleaved, and often highly calcareous shales, containing numerous nodules or thin but distinct bands of limestone. In some places the beds of limestone are the most largely developed; and in other places the shales predominate; but the two elements of the group are usually intermixed, and alternate with one another indefinitely. At some localities, again, as at Ash Gill, near Torver, and at the head of Appletreeworth Beck, the shales are largely developed at the expense of the limestone, and are sufficiently thick to have been extensively worked for slates. At Keisley, near Dufton, again, the series is almost calcareous, and the shales appear to be entirely confined to its upper portion. Finally, at Beck and Waterblain, near Millom, the group consists of an upper series of cleaved fossiliferous shales, in the lower part of which a great thickness of limestone is developed, dis continuously, in the form of great lenticular masses of a purely calcareous nature.
In its intimate characters the limestone of the Coniston Limestone group differs greatly in different localities. Usually it presents itself as a hard, compact, greyish blue, grey, or nearly black limestone, which, in thin sections prepared for the microscope, exhibits a subgranular matrix in. which fragments of Crinoids, Corals, or Brachiopods are imbedded at intervals. At Keisley (fig. 2), where we have most carefully examined it, three principal varieties may be distinguished:—(1) a hard, compact, greyish blue limestone with the microscopic characters and general aspect of the ordinary variety of the limestone just alluded to; (2) a reddish or pink compact marble with numerous patches of white calcite, both portions of the mass appearing, in microscopic sections, to be crowded with minute organisms and fragments of larger fossils; (3) a light-coloured, whitish blue or white, coarsely crystalline limestone, which is seen, in thin sections, to be composed of innumerable fragmentary organic remains and microzoa; imbedded in transparent calcite, and having large crystals of calc-spar with their characteristic cleavage-lines shooting through the mass in various directions.
Fig. 2.—Sketch Section of the Coniston Limestone at Keisley, near Dufton. (Length rather more than one third of a mile.)
|
a. Borrowdale series. |
b. Coniston Limestone. |
c. Barren ground, occupied in whole or in part by the Graptolitic mudstones.
|
d. Knock beds (pale slates). |
p. Permian strata. |
