determined by the thick dominating sandstones, which crest and form the bulk of all the loftier hills, and, along with the associated shales, give character to the high bleak moorlands ranging through the centre of Northumberland, and spreading towards the south-western borders of the county.
These Mountain Limestone strata may be separated into two groups, both having certain organic forms in common, yet each marked by differences, partly organic, and partly mineral. The upper group, which may be designated calcareous, includes all the beds, from the base of the Millstone Grit, down to the base of the Dun Limestone, the lowest limestone of any value in the formation: it has an aggregate thickness of about 1700 feet, and is chiefly distinguishable by its good workable limestones, interstratified among alternations of sandstone, shale, and coal, and by the large number of marine organisms connected with the calcareous strata. Of limestones there are upwards of twenty different beds from 1 to 30 feet thick, and having an aggregate thickness of more than 200 feet. A little below each limestone is a coal seam, or traces of coal. Some seams are of fair quality, two, the Shilbottle and the Licker coals, are of superior quality, and one, the Beadnell coal, is in some parts 5 feet thick; yet most of the others are poor and thin. This calcareous group is on the same zoological horizon as the Lower or Scar Limestone of Yorkshire, for it yields such organic forms as the following, most of which occur in the Mountain Limestone of Ireland, and many of them in strata of the same age in Fifeshire and elsewhere in Scotland.
Aulopora gigas; Chætetes septosus; Hydnopora cyclostoma; Favosites parasitica; Stenopora tumida; Lithodendron affine, junceum; Lithostrotion striatum, Portlocki; Syringopora ramulosa; Archæeoidaris glabrispina, Urii; Cribellites carbonaria; Serpulites carbonarius, Sabella antiqua: Griffithides Farnensis, the only species of a trilobite I have found; Glauconome pluma, pulcherrima; Athyris ambigua, hastata, plano-sulcata; Chonetes Hardrensis, Dalmaniana, polita; Orthis resupinata; Spirifer striatus, trigonalis, glaber; Spiriferina laminosa, octoplicata; Rhynchonella pleurodon; Streptorhynchus crenistria; Productus Cora, fimbriatus, longispinus, spinulosus, Martini; Lingula squamiformis; Aviculopecten concentrico-striatus, tus, coelatus, cancellatus, docens, duplicostata, varii-ornatus, conoides,
