Wright.] 
154 
[Dec. 19, 
Gravel and Brick Clay were deposited by the ice-laden floods which 
annually poured down the valley in the summer seasons. 
As the ice retreated towards the headwaters of the valley, the pe- 
riod was marked also by a reelevation of the land to about its present 
height, when the later deposits of gravel at Trenton took place. 
Dr. Abbott’s discoveries at Trenton prove the presence of man on 
the continent at that stage of the glacial epoch. Mr. Cresson’s dis- 
coveries prove the presence of man at a far earlier stage. How 
much earlier will depend upon our interpretation of the general 
facts bearing on the question of the duality of the glacial epoch. 
Mr. McGee of the United States Geological Survey has recently 
published the results of extensive investigations carried on by him 
respecting the superficial deposits of the Atlantic coast . 1 He 
finds that on all the rivers south of the Delaware there are 
deposits corresponding in character to what Professor Lewis had 
denominated Philadelphia Red Gravel and Brick Clay. These 
deposits are rather coarse at the bottom and fine at the top, 
though boulders two or three feet through frequently occur in the 
clay, showing that the currents of water were ice-laden at the 
height of the period. But according to Mr. McGee these deposits 
diminish both in extent and elevation as we proceed southward 
along the coast. But still the marked evidence of floating ice is 
seen in the occasional boulders enclosed in the finer material. From 
the extent to which this deposit is developed at Washington in the 
District of Columbia, Mr. McGee prefers to designate it as the 
Columbia formation. But the period is regarded by him as identi- 
cal with that of the Philadelphia Red Gravel and Brick Clay which 
Professor Lewis had attributed to the period of maximum glacial 
development on the Atlantic coast. 
It is observable that the boulders in this Columbia formation 
belong, so far as we know, in every case, to the valleys in which 
they are now found. The boulders in the Philadelphia Brick Clay 
are all from the Delaware valley, and so, about Baltimore, the 
corresponding deposits can be traced to the local valleys converg- 
ing at that point. At Washington the boulders and pebbles are 
all from the Potomac valley. It is observable also that it is not 
necessaiy in any case to suppose that these deposits were the 
direct results of glacial ice. Mr. McGee does not suppose that 
glaciers extended down these valleys to any great distance. In 
x See Am. Jour. Sci., Yol. xxxv, Feb., April, May and June, 1888. 
