CRITERIA AS TO TIME-LIMIT OF DISINTEGRATION. 329 



While in the first case described the disintegrated granitic material forms 

 the present surface soil, there are abundant street and road cuttings in 

 the northwestern part of the District where the unconsolidated sands 

 and gravels of the Potomac and Lafiij^ette formations as described by 

 Messrs McGee * and Darton are to be found overlying it at this same or 

 greater altitudes and in beds of no inconsiderable thickness. In all such 

 cases the line of demarkation between the two is well defined and there 

 is no apparent admixture of materials. 



Although both the Potomac and Lafayette gravels contain materials 

 undoubtedly derived from these older crystalline rocks, yet we do not 

 find along the line of contact an3'thing to indicate that they were laid 

 down on surfaces such as now exist or were other than fresh and hard. 

 There are included in the lower part of the gravel none of the large 

 angular masses of quartz from the veins, such as now so commonly dot 

 the surface, nor natural joint-blocks of the granite. On the supposition 

 that the beginning of the present decomposition antedates the laying 

 down of these gravels, we must assume a submergence and deposition in 

 waters so quiet as not to disturb the rotted materials. That such a condi- 

 tion is impossible becomes apparent when we consider the character of 

 the deposits tliemselves. As described, they consist of quartzite pebbles 

 derived evidently from the axial quartzites of the Blue ridge, quartz 

 pebbles identical with the vein-quartz of the region and from which they 

 were evidently derived, and a loosely consolidated arkose made up of 

 angular grains of quartz and of feldspar or flakes of kaolin, scales of 

 mica, etcetera. To this list I would add for the region about Washing- 

 ton an aljundant sprinkling of well rounded pebbles of a felsitic quartz- 

 porphyry, which, like the quartzite, was evidently derived from the Blue 

 ridge. The character of the accumulations, as 'Mr McGee states,t are — 



"Just such a.s would be formed by the assortment and deposition of the different 

 materials by * powerful currents' (author's italics), but the quantity of coarse ma- 

 terial is greater than would result from simple admixture of the disintegrated 

 gneiss of the Piedmont zone and such proportion of the Blue Ridge quartzite, 

 vein-<iuartz, etcetera, as appear to be mingled with it, suggesting that the portions 

 of the fijrmation now exposed were littoral, and that the finer materials were 

 swept into deeper, offshore waters." 



The pebbles of this formation, it should be stated, are almost invariably 

 well rounded by water-action and occur of all weights U}) to 200 and more 

 j)ounds. It seems safe to assume tliat these somewhat sporadic, larger 

 forms are due to drifting ice and for our i)resent pur[)()ses may ])e loft 

 out of consideration. 



• Am. Jour. Sci., Februnry, Marcli, April and May, 1888. 

 tOp. cit., February, 1888, p. i:». 



XLVI-Blll. Gkol. Soc.'Am.. Voi,. C. 1894. 



