Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/76

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3. The cockleshell limestone, not exceeding 20 inches in thickness, and next below the scar limestone. It is of a dark iron-grey colour, and contains besides the encrinal fossil, oyster shells of the diameter of 4 or 5 inches, and other bivalve shells. It crops out in several of the small gills on Aldstone moor, where it does not exceed in thickness 20 inches.

4. The Tyne bottom limestone, the 10th in the series, 21 feet thick. It is an encrinal limestone, consisting of 3 strata, forming the bed of the Tyne for 4 miles from Tyne head to Garigill gate, and is the lowest bed in which the mines have been wrought on Aldstone moor, though nearly the uppermost at Dufton.

5. Robinson's great limestone, the lowest in the Dufton section, and 14 fathoms thick.

6. Melmerby scar limestone, the thickest in the whole formation, measuring 21 fathoms in Melmerby cliff§ where it bassets out. It contains the encrinal fossil, and bivalve shells.

The beds of limestone have been observed to be more regular in thickness throughout the mining field than those of shale or of sandstone.

The beds of shale or plate (as it is called) are very numerous, and are found alternating with the rocks of limestone and sandstone. They are seldom so thick as 7 or 8 fathoms; but the plate sill, which is the lowest bed in the section below that of Dufton, measures 10 fathoms. Shale alternating with sandstone in thin layers sometimes forms beds of considerable thickness, (see section of Hely field,) which are called grey beds: when containing laminæ of hardstone and iron pyrites it is called a girdle bed. Iron pyrites is found imbedded in the shales in great abundance, and in various forms; but owing to the high price of fuel and the great distance from any seaport it cannot be manufactured into green vitriol to advantage. Clay