I9I2] STEVENSON— THE FORMATION OF COAL BEDS. 475 



grew a forest of Calamites, which, in their turn, were surrounded 

 by the mud, which reaches to the top of the larger Sigillaria. This 

 growth of Catamites on sand, which had buried the Sigillaria, 

 recalls the conditions at Topeka, Kansas, already described, where 

 during a great flood the young trees of a large nursery were buried 

 by sand, on which a dense growth of cottonwoods developed within 

 a few weeks. The conditions recall also those described by Russell 

 on the Yahtse River in xAlaska. The trees were killed, not broken 

 by the mass of gravels ; the trunks decayed, some were broken off 

 by the wind and the stumps were buried under new material brought 

 down by the river, but others remained, at the time of Russell's 

 examination, projecting many feet above the surface. 



The evidence all points in one direction. The buried channel- 

 ways, the cross-bedding reported at many localities and the rounded 

 pebbles indicate river not shore deposit. Plants buried in situ by 

 inundation, the ripple-marked and sun-cracked surfaces, the rain 

 prints, the footprints of batrachians, the pool-like accumulations of 

 vegetable matter, the absence of marine fossils and the distribu- 

 tion of the coarser materials make up, altogether, a mass of evi- 

 dence which it is difficult if not impossible to controvert. The sand- 

 stones were great flats, subject to inundation by the rivers to which 

 they owed their origin. There appears to be no evidence to support 

 the supposition that they are either shore or deepwater deposits. 



The Shales. — Coal Measures shales vary in structure from 

 merely compacted muds to finely or coarsely laminated beds ; in 

 composition from fine clay to sand or impure limestone ; in color 

 from almost white to black, the latter often passing to cannel or 

 even to ordinary coal. Thick deposits of shale frequently hold 

 lenses of sandstone and similar lenses of shale occur in sandstone. 

 Some shales are rich in remains of a marine, a brackish water or 

 of a freshwater fauna ; others are crowded with impressions of land 

 plants, retaining the most delicate markings of all parts ; others 

 have only indistinct plant remains, with which a marine fauna some- 

 times occurs ; others still, of notable thickness and area, have rare 

 and mostly obscure traces of either vegetable or animal life. 



