Page:A Treatise on Geology, volume 1.djvu/183

Mountain limestone formation. (Old red sandstone almost wholly absent.)

Slight representatives of millstone grit and limestone shale may be seen at the gorge of the Avon, at Bristol, round the South Wales coal field, base of the Clee hills, &c.

Further north, viz. in the north-western parts of Yorkshire, the series is still more complicated and varied: as under:—

1. Coal formation, 3000 or 4000 feet

2. Millstone grit.—A series of three mostly pebbly gritstones, separated by shales and several other flaggy, calliard and freestone grits; cherts; thin limestones; iron stones; and several coal seams. 1000 feet.

3. Yoredale rocks (equivalent of the lower part of limestone shale), a series of five or more limestones, with many freestones, flagstones, abundance of plates, some ironstone, chert, and several coal seams.—1000 feet.

4. Scar limestone, divided by partitions of grits and shales, and even some beds of coal.—800 feet.

5. Alternations of red sandstone, red clays, and limestone.—800 feet. (Red sandstone and conglomerate, very limited in their range; thickness variable. 100 feet and upwards.)

Pursuing the system to Northumberland, we find the scar limestone broken up into very many parts by inter positions of grits, shale, and abundance of coal; one of the grits being pebbly. Thus the whole method of variation of the system of carboniferous strata becomes known and appears nearly as in the diagram (fig. 18. p. 59.).

We may here notice the remarkable section presented in the Island of Arran, where, according to Murchison and Sedgwick, the new and old red formations are merely separated by a thin zone of limestone and coal, or, as from a careful examination we should be disposed to express it, where only small and diminished members of the mountain limestone formation (in one place yielding coal) appear buried in masses of red conglomerate, sandstone and shale, of very great thickness, there being no certain criterion for deciding that any of this series belongs to the new red sandstone. This