INTERPRETATION OF THE SECTIONS 493 



That the point of greatest thickness of the Shawangunk marks the de- 

 bouchure of the stream which brought the pebbles from the old land can 

 hardly be questioned. They could not have been carved from a cliff, 

 unless it were one of pure vein quartz, for otherwise the pebbles would* 

 not be of such pure character throughout. To any one familiar with a 

 modern seacoast this need not be demonstrated. Nor would the distribu- 

 tion of the pebbles be such that the deposit is thickest in one point only 

 and thins away in all directions. We should expect relative uniformity 

 in thickness parallel to the shore. Only a river deposit — that is, a river 

 delta — may have the form of a cone decreasing in all directions away 

 from the source of supply by progressive outward overlap, and likewise 

 decreasing toward the source of supply by backward overlap. The ques- 

 tion next arises, Did this deposit form in an arm of the sea or did it form 

 on dry land ? So far as the deposit itself is concerned, a sea is not neces- 

 sary. Just such a deposit could accumulate on dry land. It would have 

 all the lithic and structure characteristics found in the Shawangunk. 

 These are : 



First. The decrease in coarseness of material from the center outward, 

 and also upward, since the lowering of the grade, coincident with the 

 enlargement of the fan, would cause the coarser material to be deposited 

 farther upstream. 



Second. The abrupt changes in texture from conglomerate to sand- 

 stone and back and the irregular extent of the several members. This 

 latter feature is indeed hard to explain on any other hypothesis. 



Third. The purity of the pebbles — that is, the similarity in lithic char- 

 acter — which indicates long transportation and destruction of all but the 

 quartz pebbles. 



Fourth. The moderate rounding of the pebbles. 



Fiftji. The abundance of feldspar in the ground-mass, which on a sea- 

 shore is more apt to suffer destruction, as shown by the studies of Mackie 

 on the sands of Scotch rivers and coasts. 90 



Sixth. The combination of rounded grains, purity of sand, and com- 

 plex or eolian cross-bedding. The cross-bedding might be caused here 

 and there where a bar is just awash, but the combination of this with the 

 other characters mentioned on the seashore could only be by the merest 

 chance, while it is the expected thing in the case of eolian deposits. 



Seventh. The widespread occurrence of cross-bedding of (he torrential 

 type throughout the entire visible extent of the deposit from northeast to 



B0 Trans. Edinburgh Geological Society, vol. 7. pp. lis. 298, 143. Summarised in 

 Principles of Stratigraphy, where many other Illustrations arc given. 



