52 STEVENSON— FORMATION OF COAL BEDS. lApdizi. 



of calcareous type show that Hmestone must have been exposed 

 within the drainage area. 



The thickest underclays should belong to beds next or near 

 above the great sandrocks and it is a fact that our great clay beds 

 are near the base of the Lower Productive Coal Measures [Alle- 

 gheny] and that the few important clay deposits high in the series 

 have coarse grained sandrocks not far below them. A logical con- 

 sequence of such conditions is that sandrocks geologically close to 

 such great underclays should be purer, more open sands and gravels 

 than others which had not been robbed of so large quantity of 

 interstitial clay. If the surrounding land contained iron in its 

 gravel, there should be ball ore in the fireclay — as is seen in the New 

 England ponds surrounded by drift. 



Davis^'^ described a cannel deposit in Yorkshire, somewhat resem- 

 bling that discussed by Lesley. The bed is thickest in the center 

 and thins away in each direction, meantime becoming less pure and 

 passing into bituminous shale at the circumference. The condition 

 is due to in-floating of plant remains, which sank to the bottom of 

 the pond. The marked interlamination of shales and their marked 

 increase toward the border resulted from more rapid subsidence of 

 the muds. In some places the pond was filled up ; there the under- 

 clay has abundance of Stigmaria and the plants growing in such 

 places were converted into ordinary coal. Afterwards the whole 

 mass was submerged and covered with black mud. The cannel is 

 fine, close-grained, homogeneous, with conchoidal fracture, without 

 planes of deposition and everywhere yields beautiful specimens of 

 fishes. 



Reinsch''^-' undertook the microscopic study of coal. He pre- 

 pared a great number of sections, subjected them to close examina- 

 tion and published his results in an elaborate volume with 95 plates. 

 These exhibited the structure of the coal as well as numerous forms 

 which seemed to be organized. Reinsch maintained that the coal 



^*J. W. Davis, "On the Fish Remains found in the Cannel Coal of the 

 Middle Coal Measures of the West Riding of Yorkshire," Q. J. G. S., Vol. 

 XXXVI., 1880, p. 56. 



'-^ P. F. Reinsch, " Ncuc Untersuchungen uher die Mikrostructur dcr 

 Steinkohle des Carbon, der Dyas nnd Trias," Leipzig. 1881. 



52 



