358 PALEOZOIC TIME. 



therefore the former continuity of a single coal-bed through the East 

 and West requires strong proof, to be admitted. 



The coal-beds are thin, compared with the associated rocks. But 

 the time of their accumulation, or the length of all the periods of 

 verdure together, may have far exceeded the time that was given up 

 to the accumulation of sands and limestones. If there were but 100 

 feet of coal in all, it would correspond to between 500 and 1000 feet 

 in depth of vegetable debris. The sands and clays came in after each 

 time of verdure, to store away the product for a future age. 



These submergences, although quietly carried forward, sometimes let 

 in currents or waves of great force, as shown not only by the forma- 

 tion of coarse gravel beds (now conglomerates), but also by the 

 erosion of the rock-deposits, and also of the beds of vegetable debris. 

 In Vermilion County, Illinois, as observed by F. H. Bradley, a por- 

 tion of the Upper Coal-measures, including shales, argillaceous lime- 

 stones, and two coal-beds, were carried away to a depth of sixty feet ; 

 and, in the depression thus made, a sandstone, which belongs at the top 

 of the series, was laid down so as to fill and overlie it. Also, on the 

 same authority, in Vermillion County, Indiana (adjoining the county 

 just mentioned), the Millstone-grit (here a pebbly sandstone), under 

 the Coal-measures, is cut off short, and followed horizontally by shale 

 and limestone ; as if the grit stood as a bluff in the waters, in which 

 the latter rocks were deposited. Other evidences of erosion have been 

 described from these States, and also from Ohio. 



In Nova Scotia, the changes during the Carboniferous period (or 

 Carboniferous and Permian) went on until 14,570 feet of deposits 

 were formed ; and, in that space, as has been stated, there are seventy- 

 six coal-seams and dirt-beds, indicating as many levels of verdant 

 fields, between the others when the waters prevailed. In Pennsyl- 

 vania, there are nearly 3,000 feet of rocks in the series, above the Mill- 

 stone-grit, and 60 to 120 feet of coal. 



In the Nova Scotia Coal-measures, there is evidence in the fossils 

 that the waters were to a large extent fresh or brackish. The oc- 

 currence of a Spirorbis along with the Pupa and Reptilian remains, 

 in the Sigillaria stump, has been considered as evidence, in this par- 

 ticular case, of the presence of brackish water during the burial of the 

 stump. Onty one bed in the Nova Scotia Coal-formation, above the 

 Subcarboniferous portion, is known to contain marine fossils. The 

 land-snail {Pupa) occurs also in abed — an under-clay — over 1,200 

 feet below the level of the stump in which it was first found ; and in 

 this interval there are twenty-one coal-seams, showing, as Dawson ob- 

 serves, that the species existed during the growth and burial of at 

 least twenty forests. 



