18 FOURTH REPORT — 1834. 



the tertiary period, and not connected, as imagined by the vulgar, 

 with human agency. The usual position of these beds of Ostrea 

 is near the rivers, at a small elevation from the tide. They seem 

 to hold also nearly the same elevation along the coast of New 

 Jersey and elsewhere. 



Ancient Alluvium. — The above subdivision of our strata is 

 adopted for the sake of treating, under an independent head, a 

 group of beds of no inconsiderable extent in the United States, 

 and which, in their phsenomena, seem to cast important light 

 upon the former revolutions of the Atlantic side of the Continent. 

 They point to a period when this coast had a very different con- 

 figuration, and denote in a striking manner one of the revolutions 

 M'hich have impressed upon the tract included between the sea 

 and the mountains the peculiar features which it now bears. 

 The formation I allude to immediately underlies, wherever it 

 occurs, the general investment of diluvium. 



It has produced, hitherto, very few organic remains of the 

 description proper to enable us to judge of its relative place in 

 the series ; but as the few shells occasionally found in it belong 

 to species now inhabiting our Atlantic waters, and as, from aU 

 its other characters, it has evidently been formed imder differ- 

 ent circumstances from our other tertiary beds, and at a period 

 apparently much more recent than any of the rest with which it 

 can be compared, I am induced to place it thus apart, and to 

 give it provisionally, from its obvious origin, the convenient and 

 not too theoretical name of 'ancient alluvium.' 



This deposit is the same which has usually, in this country, 

 gone under the name of plastic clay formation, — a title suf- 

 ficiently inappropriate, even were it to express correctly its true 

 place in the tertiary series, and now particularly ineligible, when, 

 in place of being one of the lowest tertiary deposits, it will be 

 seen, from the evidence I shall present, to be one of the verj"^ 

 uppermost. The beds I am speaking of consist generally of 

 numerous alternating deposits of gravel, sand, various coloured 

 tenacious clays, often black and ferruginous conglomerates, 

 iron ore, and lignite. They occur exposed in the deeper sec- 

 tions of our canals and rail-roads, and in the banks of some of 

 the rivers, where they usually reach from the water's edge to 

 an elevation of sixty, seventy, or more feet. They extend along 

 the upper edge of the Atlantic plain, ranging along the east- 

 ern base of the rocky Atlantic slope, in a belt several miles 

 wide, and appearing at intervals, where the rivers have cut 

 through them, from the coast of Massachusetts as far at 

 least, it is believed, as the Mississippi. Professor Hitchcock, 

 speaking of these beds in the valley of the Connecticut river. 



I 



