CARBONIFEROUS AGE. 313 



The Nova Scotia Coal region abounds in erect trunks, standing on 

 the old " dirt-beds," as illustrated in Fig. 614, from a memoir by Daw- 

 son. Each of the seventy-six coal seams at the Joggins has its dark 

 clayey layer, or " dirt-bed," beneath. In fifteen of them, there is only 

 a trace of coal ; but these, as well as the rest, contain the Stigmarice, 

 and often support still the old stumps. 



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. 



The rock capping a coal-bed may be of any kind, for the rocks are 

 the result of whatever circumstances succeeded ; but it is common to 

 find great numbers of fossil plants, and fragments or trunks of trees, 

 in the first stratum. 



The shaly beds often contain the ancient ferns, spread out between 

 the layers with all the perfection they would have in an herbarium, 

 and so abundantly that, however thin the shale be split, it opens to 

 view new impressions of plants. In the sandstone layers, broken 

 trunks of trees sometimes lie scattered through the beds. Some of the 

 logs in the Ohio Coal-measures, described by Dr. Hildreth, are fifty to 

 sixty feet long and three in diameter. 



2. Coal Beds. — The thickness of the coal beds at times hardly 

 exceeds that of paper ; and again it is from thirty to forty feet. Some 

 of the larger beds may extend continuously over thousands of square 

 miles ; but, if so, they vary greatly in thickness ; and many beds thin 

 out laterally, or graduate into coaly-shales, in the course of a few scores 

 of miles. Shaly layers sometimes make up a large part of the so 

 called coal-bed. The Mammoth bed of the Lackawanna region is, at 

 Wilkesbarre, twenty-nine and one half feet thick ; while in western 

 Pennsylvania, according to the section by Lesley on page 311, the 

 thickness is but three to five feet. Where thickest, it is nearly pure 

 coal ; yet there are some black shaly layers, one to twelve inches 

 thick. The same great bed is worked at Carbondale, Beaver Meadows, 

 Mauch Chunk, Tamaqua, Minersville, Shamokin, etc. 



The Pittsburg bed, at Pittsburg, Penn., is ten feet thick ; but it is 

 made up of one foot, at bottom, of coal with pyritiferous shale ; five to 

 six feet of good coal ; and, above this, shale and coal, left as the roof 

 in working, though sometimes including one or two feet of pure coal. 

 It borders the Monongahela for a long distance, the black horizontal 

 band being a conspicuous object in the high shores, and in some places 

 containing seven or eight feet of good coal. It may be traced, accord- 



