19".] STEVEXSOX— FOR.MATIOX OF COAL BEDS. 51 



of 5 to lo degrees to an almost dead level bituminous coal bed? 

 There is no room for suggestion of crustal movement as the area is 

 too small ; equally the cavern theory is excluded for no limestone 

 underlies the horizon except at vast depth. He can see no explana- 

 tion for most of the localities except in the subsidence of a floating 

 bog, such as Lesquereux has described. On this the fine muds ac- 

 cumulated and the pool was filled. 



He was led in this connection to consider the sequence of coal 

 beds. If the Carboniferous plain consisted of a low area with 

 shallow ponds, the coal forming vegetation would conform to the 

 dimpled surface and there would be but one coal bed, intersected by 

 river channels. This plain, if continuous, would be not less than 

 i.ooo miles long by 300 miles wide [this refers to the Appalachian 

 basin]. It is very difficult to account for the submergence of this 

 continental plain to a depth of 50 feet below sealevel in order to give 

 opportunity for formation of a second bed. Yet this " slow de- 

 pression theory " may not be rejected easily, for without it, one 

 cannot conceive how^ 20,000 to 40,000 feet of palaeozoic sediments 

 could have been deposited ; the more so, since many of the strata 

 give every evidence of deposition in very shallow water. As a 

 partial alternati-ve. he suggests that the relative sea level may have 

 been changed by the filling of basins. The efi^ect of deposits by 

 great rivers and that of glaciation are discussed but no conclusion 

 is reached. 



In the preface to the Lawrence report, he attempts to explain the 

 origin of underclays. A peat bog and even a lake invaded by 

 sphagnous growth must have some water circulation due to percola- 

 tion from the surrounding land and to evaporation from its own sur- 

 face — but the movement would be very feeble and it could transfer 

 only the finest mud. though in course of time the result would be 

 important. Dry grounds are largely fine gravel with rounded quartz 

 and feldspar grains ; the feldspar is soluble, it follows the indraught 

 and settles beneath the evaporating surface with its floating peat. 

 If the peat area be surrounded by clayey land, the percolation would 

 be at a minimum ; the water supply would be from the surface 

 and less muddy, so that the underclay would be less in quantity. It 

 would appear, then, that when the margin was a tight clay, deposits 



51 



