the Middle Atlantic Slope. 371 



gravel and small bowlders ; and still further southward, as at 

 Alexandria, the superior loam becomes fine and silty, and there 

 is but an inconspicuous bed of pebbles, with no large bowlders, 

 at the base. 



Most of the materials composing the formation in the Wash- 

 ington amphitheater may be readily traced to their sources : 

 nine tenths of the larger angular and sub-angular bowlders are 

 either (1) gneiss identical with that exposed in the gorge of 

 the Potomac river within a few miles to the westward, or (2) 

 quartz undistinguishable from that of the veins intersecting 

 the gneiss ; the well rounded quartz and quartzite pebbles are 

 indistinguishable from those of the Potomac formation, and 

 indeed in some cases Potomac outliers unquestionably in situ 

 graduate insensibly into taluses which descend the slopes and 

 in turn graduate into the Columbia gravels ; the intercalated 

 layers of plastic clay and accumulations of arkose are litholog- 

 ically identical with certain characteristic phases of the Potomac 

 formation ; the sand, clay and loam sometimes resemble the 

 residuary products formed by the disintegration of the adja- 

 cent Piedmont gneisses in situ so closely as to be distin- 

 guished only by structural features ; and in all cases the petro- 

 graphic identity is unquestionably indicative of the source of 

 the material. 



The Genesis of the Deposits. — An essential element in any 

 philosophic classification of the rocks of the earth is genesis, 

 and geologic science has now reached a stage in which processes 

 and products, agencies and results, are commonly correlated, 

 and in which at least the broader classifications are genetic. 



There are recognized five principal categories of agencies by 

 which the various superficial deposits of the earth are produced, 

 viz : chemic, igneous, glacial, aerial and aqueous. 



Now on comparing the upper member of the fluvial phase 

 of the Columbia formation with the known products of each 

 of these categories of agencies, it becomes evident that the de- 

 posits were not produced by either of the first two classes of 

 agencies, since they have no distinctive features in common 

 with chemic and igneous deposits ; that they are not glacial, 

 since they are too regularly and continuously stratified, since 

 the two members are distinct in structure and composition and 

 yet intergraduate, and since the pebbles and bowlders are 

 neither striated nor polished ; that they are not aerial since the 

 materials are coarser and more continuously bedded than those 

 transported by winds ; and hence that the deposits are aqueous 

 in origin. By legitimately extending the same process of 

 reasoning it might equally be shown that they are not fluvia- 

 tile, torrential, lacustral, nor marine, and indeed that they can 

 only be a sub-estuarine delta of the river on which they occur. 



