472 J. BARRELL — ORIGIN OP THE MAUCH CHUNK SHALE 



A possible explanation of the phenomenon in some instances may be as 

 follows : Elvers which over a portion of their course are aggrading 

 streams are commonly only such for a portion of the year. During the 

 balance of the year the water is less in quantity, is underloaded, therefore 

 is degrading, and flows within channels which carry the sediment through 

 to the sea. The size of the cliannel is doubtless determined to some 

 extent by the volume of the stream during its season of degradation. 

 Where the discharge is always large, as in the Mississippi system, large 

 permanent channels through the floodplain will exist; where small, as in 

 the Platte, the result may be a shallow channel holding a braided river, 

 which, to use a popular aphorism, is a mile wide and a foot deep. Where 

 there is no water except during cloudbursts, and every flood is overloaded 

 with debris, sheetflood erosion and deposition, as described by McGee, may 

 be the consequence.* 



In a region of arid or semiarid climate, therefore, where the rainfall is 

 highly concentrated and the streams are either dry or overflowing, there 

 would seem to be a favorable case for the development of interlaminated 

 sand and clay deposits without the occurrence of massive sandstones pos- 

 sessing an irregular bottom and cross-bedded structure. That the explana- 

 tion may not be so simple as the above is indicated, however, by the deep 

 scouring of such streams as the Platte, which in times of flood may "rise 

 downward" as much as 90 feet, and which by the wanderings of the 

 channel must rework the thin-bedded deposits of its floodplain and pre- 

 sumably leave sand in the fiUed-up jjlaces. Over a broad delta, however, 

 such deep scouring would not appear to be called for, since there the 

 flood waters cover a large part of the country and move leisurely seaward, 

 relieving the channels from carrying an undue proportion of the water. 

 Under such conditions, also, the channels are insignificant in area in 

 comparison with the floodplain, and consequently in the lateral wander- 

 ings of the channels accompanying the upbuilding of the floodplain but a 

 small per cent of the deposit would represent the fillings of the aban- 

 doned courses of the larger distributaries. 



Organic evidences as to climate. — At times as early as the Middle 

 Devonian an arboreal vegetation existed where the climate was suitable. 

 In the sub-Carboniferous, and presumably earlier, the vegetable kingdom, 

 then as now, lived in certain groups adapted each to a different environ- 

 ment. These plant societies at that time, however, were made up entirely 

 of gymnospermous and crj^ptogamic forms, plants which have since been 

 driven into the background by the more successful class of angiosperms. 

 While it would be unsafe to argue too rigidly that the identical conditions 



• Bull. Geol. Soc. Am., vol. 8, 1897, pp. 87-112. 



