PALjEoNTOLOGICAL report of geological survey. 511 



des, . (plate 7, fig. 2.) which undoubtedly, like the Char a and the 

 Horsetail of our time, has especially contributed to fix the silica, and 

 to precipitate it to the bottom with its remains. 



In this abridged exposition we cannot discuss the value of any of 

 the above made assertions. -Nevertheless, not one of them has been 

 admitted without a critical ^examination, and after its trnth has been 

 ascertained by serious researches, or by reliable authorities. 



The formation of the coal being thus understood in its whole, it is 

 easy to draw from it the explanation of the different modifications of 

 the Coal Measures, and to deduce some general rules for the identifica- 

 tion of the veins. 



1st. the fire-clay. 

 This clay, ordinarily full of rootlets and stems of stigmaria, so gen- 

 erally underlays every bed of coal, and its general appearance and 

 chemical elements are so much the same, that except, perhaps, for its 

 general thickness, it cannot become a very reliable guide for the iden- 

 tification of the beds. Even its thickness is variable. It depends on 

 the depth of the basin in which it is formed, and on the regularity of its 

 bottom — thickening in the hollows, and sometimes entirely disappear- 

 ing near the margins of the basin. Variously tinctured by more or 

 .. less- of oxide of iron, it is generally whitish, but sometimes as red as 

 ochre, and even variegated like marble, in the same bed. The quan- 

 tity of stigmaria found in it is as variable as its color, and as for its 

 chemical elements they depend, like the color, on the mixture of iron 

 and lime, especially silica and alumina, which are never uniformly dis- 

 tributed in a wide expanse of shallow water. This fire-clay of the Coal 

 Measures appears sometimes alone, and without any bed of coal above 

 it. In which case it may be intermixed with layers of shales, covered 

 with the remains of plants, especially of ferns. Then it indicates only 

 the place which was prepared for the vegetation of a bed of coal. 

 Some accident — the shallowness of the water perhaps, or some dis- 

 turbance of its level — has prevented the growth and accumulation of 

 vegetable matter in sufficient abundance to form the coal. But the 

 plants, growing upon the marsh, have been imbedded and preserved in 

 the shales above the fire-clay as testimony to its natural destination. 

 Nevertheless, those isolated beds of fire-clay, overlaid by plants, are not 

 always barren of coal, and by following them to some distance the coal 



