the Middle Atlantic Slope. 369 



too intimate to be fortuitous. Everywhere west of the Ana- 

 costia-Potomae channel (the deposit does not occur east of the 

 rivers) the limiting boundary of the fluvial phase of the forma- 

 tion is the 150 foot contour; and the limit within which it ex- 

 hibits a certain notable and well defined type is the 90 foot 

 contour. 



Within the amphitheater the formation varies considerably 

 in structure and composition and in the relative thickness of 

 the two members — the basal member being best developed 

 centrally and near the entrance to the Potomac gorge, and the 

 superior and finer member reaching the best development and 

 greatest volume peripherally and at points distant from the 

 gorge. The section in the central part of the city of Washing- 

 ton is, however, typical ; and two exposures so located, which 

 together form a general section of the fluvial phase of the for- 

 mation, are shown in the accompanying plates. 



Plate VI is reproduced mechanically (by the Moss process) 

 from a photograph of the exposure on the north side of E street 

 between 1 and 2 southeast. The upper member is homogeneous 

 loam, rather too sandy for use as a brick clay, either massive or 

 obscurely stratified, containing a few small pebbles irregularly 

 disseminated or arranged in layers. On mechanical analysis 

 the loam is found to consist of (1) fine silty or clayey particles 

 of impalpable fineness, intimately mingled with (2) sand grains 

 of variable size, form and composition, and with (3) gravel of 

 all sizes from that of coarse sand to that of the pebbles shown 

 in the plate ; the relative proportions being perhaps 50 per 

 cent of impalpable clay and silt, 35 or 40 per cent of sand 

 grains up to -§• inch in diameter, and the balance gravel grains 

 and larger pebbles. The homogeneous loam graduates down- 

 ward imperceptibly into obscurely stratified sandy and gravelly 

 loam in which sand from -§- inch downward constitute some 40 

 per cent, gravel from ■§■ inch upward about 30 per cent, pebbles 

 from an inch npward perhaps 20 per cent, and impalpable silt 

 not more than 10 per cent of the volume — the structure re- 

 maining unchanged save that the stratification becomes more 

 and more distinct toward the base. The gravelly loam grad- 

 uates in turn into a bed of stratified sand and fine gravel, 

 sometimes cross-laminated, with occasional pebbles up to 3 or 

 4 inches disseminated through it. This bed is practically des- 

 titute of impalpable silt, and is screened for building sand. The 

 stratified sand passes rather abruptly, but with some interstrati- 

 fication, into a heterogeneous mass of coarse sand, gravel, 

 pebbles and bowlders up to a foot in diameter. 



Plate VII, also reproduced mechanically from a photograph, 

 supplements Plate VI. The exposure occurs on the opposite 

 side of the street and, extending nearly to the base of the Co- 



