186 BRIGHAM—TOPOGRAPHY AND DEPOSITS OF MOHAWK VALLEY. 
sylvania region. Itis also assumed that the Hudson valley is a product 
of Tertiary erosion,* and that there was a Laurentian valley before the 
elevation of the Cretaceous peneplain. Westgate points out that the 
Laurentian drainage is of the subsequent order, and that the adjustment 
by which waters originally flowing southward were led to course along 
the strike of the Paleozoic beds, may have taken place in the Permian- 
Cretaceous interval.f It is assumed in this paper that the southern 
Adirondack streams and Susquehanna headwaters may be truly con- 
sidered as descended, without serious modification of location or direc- 
tion, from streams consequent on Paleozoic topography.t It is to be 
also remembered that the Mohawk valley is near and roughly parallel | 
to the pre-Cambrian-Paleozoic contact. This contact line is irregular, 
both on account of Adirondack folding and the Mohawk valley faults. 
Overlying the relatively thin Calciferous and Trenton, and less subject to 
the irregularity produced by faulting, is a thick but yielding mass of 
Utica and Hudson River shales. Given therefore the growing Hudson 
valley and a steep upland on the west, with the tough Adirondack masses 
on the one hand and the hard Helderberg and Hamilton terranes of east- 
ern New York on the other, it was to be expected that a valley would be 
formed with headward cutting westward along the Utica-Hudson strike. 
But for the irregularities above noted the river would almost surely have 
kept the north boundary line of the Paleozoic, as is conspicuously the 
case with Black river, which flows for 50 miles with slight divergence 
from this line. Dr Bell, of the Canadian Geological Survey, has made 
some interesting observations as to the effect of glaciation on such a con- 
tact.§ Apparently the Mohawk headed westas far as Little Falls. The 
Mohawk may therefore be considered a monoclinal valley, though struct- 
ural irregularities and inequalities of glacial erosion have somewhat dis- 
guised this chief character. In harmony with this view, Dana many 
years ago noted that the southern plateau was higher than that lying to 
the north.|| He, however, defined the valley as “ geoclinal.” In similar 
fashion, as it seems to the writer, a subsequent valley was formed from 
the ancient Laurentian valley, heading to Little Falls. West of Rome 
it may have passed along the axis of Oneida lake or farther south. In 
any case the form of the Iroquois basin on the east appears to be due to 
* W. M. Davis: The Catsixill delta in the Postglacial Hudson estuary. Proc. Bost. Soc. Nat. Hist., 
November, 1891, p. 319. 
+ Lewis G. Westgate : The geographic development of the eastern part of the Mississippi drain- 
age system. Am. Geologist, April, 1893, p. 245. 
{ On the probable character of such topography see Chamberlin and Salisbury: Driftless area of 
the upper Mississippi valley. Sixth Ann. Rep. U. 8. Geol. Survey, p. 224. 
2 Robert Bell: On glacial phenomena in Canada. Bull, Géol. Soc. Am., vol. 1, p, 296. 
| J. D. Dana: On the existence of a Mohawk Valley glacier in the Glacial epoch, Am, Jour, Sci., 
2d,series, vol, Xxxv, 1863, pp, 243-249, 
