90 The American Geologist. August, v.m 
formation if found isolated; Init in the Sixteenth street expos- 
in-e the two deposits are juxtaposed and sej)arated by a well- 
defined unconformity — ;". c., the stratigraphy shows that the de- 
posit in question is materially older than the Earlier Columbia. 
On comparing- the deposit with the Lafayette, as displayed in 
the nearest exposures of that formation on the west, north and 
east, it is found to be so different in materials and structure as 
to demand separation on lithologic grounds ; moreover, the de- 
posit is confined to a depression, or amphitheatre, which did 
not exist at the time of Lafayette deposition, but was pro- 
duced during the period of rapid degradation accompanying the 
post-Lafayette uplift ; so that it must be discriminated from 
the Lafayette on the l^asis of homogeny as well as on that of 
lithology. The interpretation of the deposit is simple : it is ev- 
idently a record of an oscillation during the post-Lafayette and 
pre-Columbia time, which was not of such amplitude and length 
as to inscribe itself deeply in the local series of formations and 
land-forms. On seeking to correlate the deposit with other ele- 
ments in the coastal-jilain series, difiiculty is encountered ; no 
corresponding deposits are known either southward or eastward 
in Virginia and Maryland ; the nearest known deposits of cor- 
responding character and position are a part of those found in 
southern New Jersey, and first grouped by Salisbury under 
the designation Pensauken, but afterward xlivided." 
Pressure of other duties prevented Mr. McGee from con- 
tinuing the investigations which he had so happily commenced, 
and Mr. N. H. Darton, of the United States Geological Survey, 
took and carried forward the work where Mr. McGee left it. ' 
VIEWS OF MR. N. H. DARTON. 
According to Mr. Darton, the Lafayette formation does not 
end at Fredericksburg, but in disconnected areas, crosses Mary- 
land, Delaware and Pennsylvania, into New Jersey. In this 
connection he said :* "The northern termination of the deposits 
was supposed to be near Potomac creek, a few miles north of 
Fredericksburg ; but I have found that while there is a break in 
its continuity in the region east of the Potomac river, it soon be- 
gins again and thence continues northward probably through 
Maryland, and in attenuated scattered outcrops, through Dela- 
ware and into Pennsylvania and New Jersey. It is displayed 
in the high terraces about Washington, and it caps nearly all 
