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



The observations by Firket are not without interest but, as he 

 recognized, they have little bearing on the matters at issue here. 

 Shales ofttimes are merely laminated clays and lose their lamination 

 v^hen exposed to the atmosphere. There are many roads in the 

 Appalachian basin which show deep through cuts in argillaceous 

 shale. Less than a century, frequently much less than half a cen- 

 tury has passed since the roads were constructed, yet the period has 

 sufficed for conversion of the outcrops into plastic clay. But that 

 is not the question. The Lower Kittanning coal rests on a bed of 

 plastic clay, lo to 20 feet thick, an excellent potters clay, used in 

 manufacture of various wares along a line of more than 150 miles in 

 Pennsylvania, Ohio and West Virginia; a flint clay at the base of the 

 Allegheny, 5 to 25 feet thick, is utilized at many places along a line 

 of fully 100 miles in Maryland and Pennsylvania. No condition 

 such as that described by Firket seems likely to afiford even a sugges- 

 tion toward explaining the accumulation of such deposits, which, 

 except as to thickness, are typical. Nor can one find sufficient ex- 

 planation for the small proportion of iron in activities of plant life, 

 since those could afifect only the superficial portion. The features 

 seem to be original in the mass and due to the work of atmospheric 

 agencies prior to deposition. Long exposure of rocks causes deep 

 distintegration and decomposition, as has been proved by Russell 

 Crosby and Belt, already cited in another connection. The widely dis- 

 tributed Kittanning clay followed the Vanport subsidence, which had 

 been preceded by a long period of quiet or of local elevation, during 

 which deep valleys were eroded on the west side of Alleghania and, 

 in an extended area, no new deposits were laid down. When the 

 disintegrated materials were removed, the finest clays were deposited 

 by themselves, carrying with them the impalpable humus of the 

 soils. The strange irregularities, exhibited by beds in the closing 

 portion of the Beaver, are evidence of a similarly long exposure for 

 great areas and afford reason for applying the same explanation to 

 the other great deposit. The condition may have been similar else- 

 where and may account for clays under coal beds as well as at 

 horizons where deposition of clay was not followed by conditions 

 favoring accumulation of coal. 



