3T0 IF. J. McGee — Three Formations of 



lumbia formation, exhibits the typical aspect of the lower 

 member — the greater part of the superior loam having been 

 artificially removed long before the recent excavation was 

 made. The uppermost stratum consists of pebbly and sandy 

 loam corresponding to but somewhat coarser than the basal por- 

 tion of the upper member in Plate YI ; and in this section, too, 

 the stratum graduates insensibly into stratified sand, which in 

 turn passes imperceptibly into the gravel bed at the summit of 

 the lower member. 



The gravel deposit constituting the lower member is dis- 

 tinctly but irregularly stratified and rather indefinitely tripar- 

 tite. The uppermost stratum is a bed of gravel and sand 

 similar to but thinner than that above the stratified sand, con- 

 taining rounded and sub-angular bowlders up to over a foot in 

 diameter (commonly arranged in beds), lenticular layers and 

 pockets of sand, etc. ; the next stratum is a regularly bedded 

 mass of clay and loam, evidently derived largely from the 

 Potomac formation, which is locally inclined ; and finally at 

 the base there is another bed of gravel (imperfectly shown in 

 the plate) resting on an irregular surface of purple-brown 

 Potomac clays. Combining the three strata and analyzing 

 their constituents, it is found that perhaps 20 per cent consist 

 of pebbles and bowlders from an inch to a foot or more in 

 diameter, some 25 per cent of gravel and pebbles from -§- inch^ 

 to one inch in diameter, about 30 per cent of finer sand, and 

 the remainder (including the redeposited Potomac clay) of 

 impalpable silt or clay ; and examination of the pebbles and 

 bowlders shows that nearly all of the larger are angular or sub- 

 angular and either of Piedmont gneiss or of quartz undis- 

 tinguishable from the vein quartz of the Piedmont zone, while 

 75 or 80 per cent of the smaller are well rounded and of quartz 

 and quartzite similar to those of the lower member of the Po- 

 tomac formation. 



These sections occur about three miles southeast of the gap 

 cut by the Potomac river in the Piedmont escarpment, and in 

 the line of the old outer gorge. Nearer the gap the superior 

 member attenuates, and the gravel bed thickens and becomes 

 coarser until in some sections fully one-half of the formation is 

 made up of pebbles and bowlders up to four or five feet in diam- 

 eter ; to the eastward the loam increases in thickness and homoge- 

 neity, its pebbles disappear, and the stratification becomes more 

 regular, while the basal member attenuates and the pebbles 

 and bowlders of which it is composed diminish gradually both 

 in size and abundance ; to the southward and further from the 

 gap the upper portion of the loam is a homogeneous brick 

 clay, its lower portion is a stratified sand, and the lower mem- 

 ber of the formation is represented only by a thin bed of 



