48 STEVENSON— FORMATION OF COAL BEDS. [April i8, 



deposition of the clay, which filled the interstices, so that they may 

 be sought in thin deposits or at the bottom of those which are 

 thicker. 



Underclays are often very light in color and many of them con- 

 tain little iron and less carbon ; but some iron is always present even 

 in the most refractory. There is similar variation in the content of 

 alkalies. The absence of iron is believed to be due in chief part to 

 decaying vegetation. The deep red shales of the Coal Measures con- 

 tain little organic matter, few traces of plants or animals. That 

 organic acids, formed during decomposition of vegetable materials, 

 give somewhat soluble salts with iron has been known for a long 

 time, as was shown on earlier pages where are recorded the results 

 obtained by A. A. Julien and others. Miller,^^ in describing the 

 Boulder Clay of Cromarty, Scotland, gave a local illustration. On 

 the flat moor upland, where the water stagnates over a thin layer of 

 peaty soil, chance sections exhibit the underlying clay spotted and 

 streaked with grayish-white patches. There is no difference between 

 these patches and the red mass in which they occur, all alike con- 

 sisting of mingled arenaceous and aluminous particles. The stagnant 

 water above, acidulated by its vegetable solutions, seems to be con- 

 nected with these appearances. In every case, where a crack gives 

 access to the oozing moisture, the clay is bleached for several feet 

 downward to nearly the color of pipe clay. The surface, too, wher- 

 ever divested of the vegetable soil, presents for yards together the 

 appearance of sheets of half bleached linen. Dawson^* observes that 

 underclays have the white aspect which one sees in the subsoil of 

 modern swamps, and he thinks that the cause is the same in both 

 cases — the removal or transportation of ferruginous coloring matters 

 by the deoxidizing or dissolving action of organic acids or of 

 organic materials in decomposition. 



Stainier^° has taken exception to this statement of the conditions 

 and has shown that of 150 specimens of Begian underclays, barely a 



'' H. Miller, " The Cruise of the Betsy," Boston, 1862, p. 357. 

 '*J. W. Dawson, Quart. J own. Geol. Soc, Vol. X., 1854, p. 14. 

 ^° X. Stainier, " Notes sur la formation des couches de charbon," Btill. 

 Soc. Edge Geol. Vol. XXV., iqtt, P. V., pp. 73-91. 



