191-'] STEVENSON— THE FORMATION OF COAL BEDS. 471 



glomerates mark times when activity of denuding agencies was 

 greatly increased, when the destructive effect of floods upon vegeta- 

 tion ought to be most marked. 



Surface features of sandstones received Httle attention from 

 earlier observers in the Appalachian basin. Ripple marks, sun 

 cracks and rain prints seem to liave been regarded as mere common- 

 places and the recorded observations are merely incidental. Ob- 

 servers in recent years have recognized the importance of these 

 surface markings. 



The ripple or wave marks on some Silurian sandstones attracted 

 attention long ago, as they are exposed in broad spaces and, for the 

 same reason, occasional references are to be found to similar mark- 

 ings on some Devonian beds. The earliest note respecting their oc- 

 currence in Carboniferous rocks in that by Rogers,*^ who says of the 

 sandstones of Formation X that "beautiful ripple markings are 

 often met with on the surface of the large slabs of the finer of 

 these sandstones." This formation is equivalent to the Upper 

 Pocono of Pennsylvania ; and the observation is important because 

 it was made in the area where the earliest coal beds were formed. 

 The rocks are in the lowest division of the ]\Iississippian or Lower 

 Carboniferous. 



H. D. Rogers, J. P. Lesley, B. Halberstadt and G. P. Grimsley 

 have recorded observations of wave marks and mud cracks in the 

 Alauch Chunk of Pennsylvania and West Virginia ; and J. Barrell 

 has discussed their importance in a memoir to which reference will 

 be made on another page. 



Scattered observations show that such surface markings are 

 of common occurrence in Pottsville rocks.*" Smith obtained a fine 

 slab of sun-cracked sandstone of New River age near Huntsville, 

 Alabama ; Ashley reports sun cracks in New' River sandstones of 



" W. B. Rogers, " Report of the Geological Survey of the State of 

 \'irginia for 1837," reprint, Boston, 1884, p. 183. 



'^E. A. Smith, letter of December 23, 191 1 ; G. H. Ashley, letter of 

 October 24, 1911 ; J. W. Foster, cited by J. P. Lesley in " Manual of Coal and 

 its Topography," Philadelphia, 1856, p. 105; L C. White, Sec. Geol. Surv. 

 Penn., Rep. Q3, 1880, pp. 194, 195. 



PROC. AMRR. PHIL. SOC, LI, 207 D, PRINTED DEC. I4, I9I2. 



