CARBONIFEROUS PERIOD. 367 



Should a general submergence be proved, it remains a question 

 whether the lowering of the sea-level on the land was due to a 

 rising of the land, or a deepening of the ocean's bed causing a with- 

 drawal of the waters ; for the ocean's bed has ever been as liable to 

 oscillations as the continental part of the crust, and the effects 

 should have been as much greater than those from the oscillating 

 land as the area of the ocean is greater. Whichever be the mode, 

 the movements would generally have been such as Would become 

 appreciable only by the lapse of many years or a century. 



In Nova Scotia these changes went on until 14,570 feet of deposits 

 were formed ; and in that space, as has been stated, there are 76 

 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 3000 feet of rocks in the series, and CO to 

 120 feet of coal. 



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. 



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

 that the waters in which were accumulated the rocky layers that 

 intervene between the coal beds were, to a large extent, fresh or 

 brackish. The occurrence of a Spirorbis along with the Pupa and 

 Rej)tilian remains in the Sigillaria stump has been considered as 

 evidence in this particular case of the presence of brackish water 

 during the burial of the stump. There are but few beds in the 

 whole thickness of the Nova Scotia Coal formation that contain 

 marine fossils. The land-snail (Pupa) occurs in another bed — an 

 under-clay — over 1200 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 observes, that the species existed during the 

 growth and burial of at least twenty forests. It proves the terres- 

 trial character of the coal vegetation. 



In the Interior Continental region, the submergence attending the 

 formation of these intervening rocks was mostly or wholly marine ; 

 for all the fossils thus far observed are those of marine species, 

 and they occur in many strata of limestone, sandstone, and shale 

 throughout the Coal measures. Over the great Mammoth bed of 

 Wilkesbarre there are shales (at the township of Hanover) con- 



