326 



PALAEOZOIC TIME CARBONIFEROUS AGE. 



Besides the rocks mentioned, a buhrstone occurs in beds several 

 feet thick, in Ohio. It is a cellular, flinty, siliceous rock, valued 

 highly for millstones. 



The limestones are more extensive in the Coal measures of the 

 Mississippi basin than in those of Pennsylvania and Virginia ; 

 while, on the contrary, conglomerates are much less common in 

 the West. This accords with the fact, learned from the earlier 

 ages, that the Appalachian region is noted for its conglomerates 

 and sandstones, and the Interior basin for limestones. 



In Wayne co., in western Pennsylvania, there are 80 feet of limestone in 350 

 feet of Coal measures ; and near Wheeling, on the Ohio, twice this thickness of 

 limestone. In Missouri, there are 150 feet or more of the former to 650 of the 

 latter. In the lower 150 feet of the Missouri section there are, however, but 

 8 feet of limestone, and in 900 feet of the Lower Carboniferous in western Ken- 

 tucky, only 10 feet. The limestones included among the strata appear often [o 

 have a limited lateral range, instead of the uniformity over extended areas 

 common in earlier periods. 



The rock underlying a coal bed may be of either of the kinds 

 mentioned ; but usually it is a clayey layer (or bed of finer clay) 

 which is called the under-day. This under-clay generally contains 

 fossil plants, and especially the roots of Carboniferous plants called 

 Stigmarice, and it is regarded (as first shown by Logan) as the old 

 dirt-bed in which the plants grew that commenced to form the 

 coal bed. In some cases trunks of trees rise from it, penetrating 

 the coal layer and rock above it. • 



The Nova Scotia Coal region abounds in erect trunks, stand- 

 ing on their old dirt-beds, as illustrated in fig. 560 (by Dawson). 



Fig. 560. 



Section of Coal Measures at the Joggins, Nova Scotia (with erect stumps in the sandstone, 

 and rootlets in the under-clays). 



Each of the 76 coal seams at the Joggins has its darker clayey 

 layer, or dirt-bed, beneath. In 15 of them there is only a trace of 



