SURFACE OF THE MAUCH CHUNK DELTA 469 



one which is hot and dry the intense evaporation will speedily dry up 

 flat lands which otherwise would remain as swamp areas. Observation of 

 present deltas indicates, however, that even in arid climates the portion 

 marginal to the sea is occupied by permanent lagoons in which, under 

 conditions of high aridity, are deposited gypsum and salt. In climates 

 marked merely by dry seasons, however, such marginal swamp areas may 

 hold fresh or brackish waters and be the seat of a dense swamp vegeta- 

 tion. Evidences of a state approaching this is seen in the Mauch Chunk 

 shales in their outcrop beyond the Alleghany escarpment in western and 

 northern Pennsylvania, where, as noted previously, thin beds of coal 

 sparingly occur and considerable deposits of earthy carbonate of iron 

 similar to that of the Coal Measures. Taking for analogy the grade of 

 the loaded rivers flowing east from the Kocky mountains through the 

 semiarid to subhumid climatic belts, it seems probable that in the region 

 of the anthracite basins the river grades were certainly over 2 feet per 

 mile, possibly over 5 feet per mile, flattening out in the more distal por- 

 tions of the river system. 



The profile of a graded river depends upon the fineness of load and its 

 ratio to the water. This is partly dependent, therefore, upon the climate 

 as well as the lithology and topography of the regions of erosion. The 

 climatic reasons for comparing the stream grades with those of the rivers 

 flowing east from the Rockies are more fully developed in the following 

 section. 



• INFERENCES AS TO CLIMATIC CONDITIONS 



Introductory statement. — CUimate exercises a controlling influence upon 

 the characteristics of delta deposits, as may be readily seen by comparing 

 the delta of the Indus, largely a barren surface of drifting sands, with 

 that of the Amazon, sAvampy and mantled with an impenetrable forest; 

 or that of the Yukon, covered with a mat of cryptogamic and gymnospei- 

 mous vegetation, giving rise to widespread accumulations of peat. A 

 full discussion of the "Relations of climate to terrestrial deposits" led so 

 far afield that the subject was separated from the present paper and is to 

 be published under that title elsewhere.* All present arguments as to the 

 climatic inferences from the nature of a continental formation are there- 

 fore Tcmch abridged. 



Chemical and structural evidences as to climate. — In northern and 

 western Pennsylvania thin coal beds and iron carbonate occur in the 

 Upper Mauch Chunk shales, while in eastern Pennsylvania horizons of 

 conglomerate give a 'character transitional to the Pottsville. Although it 

 is seen that the coal seams and iron ore are deposited where the river 



• See abstract, Proc. Bull. Geol. Soc. Am., vol. 18, 1906. 



