KOETHTJMBEELAND AND DURHAM. 9 



determined by the thick dominating sandstones, which crest and 

 form the bulk of all the loftier hills, and, along with the asso- 

 ciated shales, give character to the high bleak moorlands ranging 

 through the centre of ISTorthumberland, 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 Bun 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, inter- 

 stratified 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 

 ia Scotland. 



Aulopora gigas ; Chtetetes septosus ; Hydnopora cyclostoma ; Favosites 

 parasitica ; Stenopora tiimida ; Lithodeudron afBne, junceum ; Lithostrotion 

 striatum, Portlocki ; Syi'ingopora ramulosa ; Ai-chfeocidaris glabrispina, 

 Urii ; Cribellites carbonaria ; Serpulites carbouarius, Sabella antiqua : Grif- 

 fithides Farnensis, the only species of a trilobite I have found ; Glauconome 

 pluma, pulcherrima ; Athyris ambigua, hastata, piano -sulcata ; Chonetes 

 Hardi-ensis, Dalmaniana, polita ; Orthis resiipinata ; Spirifer striatus, trigo- 

 nalis, glaber ; Spiriferina lamiiiosa, octoplicata ; Rhynchonella pleurodon ; 

 Streptorhynchus crenistria; Productus Cora, fimbriatus, longispinus, spiuu- 

 losus, Martmi ; Lingula squamiformis ; Aviculopecten concentrico-striatus, 

 tus, coelatus, cancellatus, docens, duplicostata, varii-ornatus, conoides, 



