74 Prof. Sedgwick on the Carboniferous Chain 



are only identified by their relation to the other groups^ and by beds of the 

 Producta latissima and P. Scotica which follow them. 



4. Group of Sandstone, Shale, and Calcareous Grit, &fc. Maximum thick- 

 ness 150 feet; average thickness about 60 or 70 feet. 



The arrangement of the subordinate parts of this group is extremely vari- 

 able^, as the different masses frequently seem to replace each other. In an 

 approximate way the group may be subdivided as follows. 



(«.) Dark-coloured shale, with one or two bands of crow-limestone or cal- 

 liard. 



(6.) Hard beds of brownish, siliceous grit, generally of a somewhat slaty 

 texture; containing subordinate, strong, hard beds of a calciferous grit, some- 

 times ferruginous. 



(c.) Alternating bands of hard, brown, flaggy grit and shale. 



The beds of calciferous grit are extremely characteristic of this group, and 

 are sometimes so expanded as to form a prominent part of it. Within they 

 are of a greyish colour ; but their weathered surfaces are generally brown, 

 from the presence of iron. The quantity of calcareous matter contained in 

 them is very variable, as they pass at one extreme into pure, close-grained, 

 siliceous grit ; and at the other, where organic remains are abundant (which 

 is however rarely the case), into a nearly pure limestone. 



The crow-limestone or calliard is not so characteristic, as it is, here and 

 there, subordinate to all the great masses which alternate with the six groups 

 of limestone. It may however be proper to explain these synonymous pro- 

 vincial terms, as they will be frequently used in the following details. They 

 are applied to a very impure variety of shelly limestone which is sometimes 

 associated with gritstone, and then passes irregularly into calciferous grit, 

 but most frequently is subordinate to the beds of dark-coloured shale. In the 

 latter position it sometimes appears in the form of concretions ; but more fre- 

 quently in irregular bands, seldom more than one or two feet in thickness. 

 The outer surface of this crow-limestone is very irregular, and is generally 

 covered with a thin, eartliy coat of hydrate of iron. It is very ponderous, in 

 parts reaching the specific gravity of the nodules of argillaceous iron ore. It 

 is difficult of fracture ; but when broken almost always shows a bluish, cal- 

 careous interior; and rarely has a core of nearly pure limestone. 



It contains innumerable organic remains, especially large specimens of a 

 species of Producta; and from the crescent-shaped impressions of that shell, 

 the miners sometimes designate it by the name of horse-shoe limestone*. It 



* The specific characters are generally obscure ; but I believe the large Products belong chiefly 

 to the species Producta Scotica, 



