191-'] STEVENSON— THE FORMATION OF COAL BEDS. 469 



cross-bedded sandstone with shale. Where the sandstones extend 

 over broad areas, the conditions are less quickly apparent; but the 

 channel-ways of streams across the often flooded plains are marked 

 by the lines of coarse materials, while the migrations of those 

 streams are recorded in pebbly bands at various levels, now on one, 

 now on the other side of the more or less persistent channel- way. 

 The form of the pebbles and of the sand grains is that due to the 

 action of running water ; the remarkable freedom from argillaceous 

 matter characterizing so many sandstones and conglomerates could 

 be brought about only by running water supplemented, perhaps, by 

 the winds. 



Other features of the sandstones deserve consideration. 



Tree-trunks have been observed in sandstones of all ages since 

 the Devonian. Long ago J. Hall found fragmentary stems in the 

 Devonian of New York; Dawson reported them from rocks of the 

 same age in Canada ; Sherwood saw them in northern Pennsylvania 

 and Newberry described forms from the Lower Devonian of Ohio. 

 Such stems are not rare in the Upper Pocono (Logan) of Pennsyl- 

 vania; they have been reported from the New River beds of Ohio, 

 the Beaver beds of Pennsylvania, Ohio and West Virginia; the 

 Millstone Grit of New Brunswick; the Allegheny beds of Penn- 

 sylvania and West Virginia and occasionally from sandstones of 

 higher formations. They are characteristic of Carboniferous sand- 

 stones in other areas as well as of those of other times. ^"^ 



In fine grained sandstones one usually finds only small pieces 

 and comminuted fragments of wood but occasionally a large almost 

 uninjured stem has been found, resembling those sometimes dropped 

 on overflow plains 'by receding floods. Large trunks are not re- 

 ported from many localities but they have been seen most frequently 

 in coarse sandstones. Hildreth saw some more than 30 feet long 

 in a Beaver sandstone on the Kanawha River ; another stem, in 



'*'S. P. Hildreth, Anter. Jour. Sci., Vol. XXIX., pp. 22, 37, 73, 76, 107; 

 L. Lesquereux, Geology of Pennsylvania, 1858, Vol. II., p. 840; F. Piatt, 

 Sec. Geol. Surv. Penn., Rep. H, 1874, p. 23; A. Sherwood, ibid., Rep. G, 

 1878, pp. 21, 38-40; I. C. White, ibid.. Rep. Q, 1878, p. 203; Rep. Q2, 1879, p. 

 137; U. S. Geog. Surv. W. of looth Mer., Vol. III., Supp. 1881, p. 196. 



