Correspondence. B45 
hawk which were plainly formed in the same water body. The upper 
Hudson is by far the largest stream descending from the Adirondacks 
and would naturally build a correspondingly large delta. 
At Rome the level of the Iroquois beach projected into the Mohawk 
valley appears to find a perfect continuation in the water-plane in which 
the terraces were formed. If the Mohawk was a marine estuary and had 
free connection with the Iroquois basin on the level of its great beach, 
it seems natural to conclude that the Iroquois beach is also of marine 
origin. No doubt a great glacier-dammed lake once filled the Ontario 
basin and had a river outlet past Rome down the Mohawk. But a ma- 
rine invasion of later date may have obliterated its marks and built up 
over them the present estuarine deltas of the Mohawk and the Iroquois 
beach. The fall of 100 feet in the old water level from Rome to Schen- 
ectady is not necessarily the fall of a river, but is probably a part of 
the general ncrthward rise, or a mere local variation. What I have seen 
thus far indicates that the terraces of the Mohawk are estuarine deltas 
and not true river terraces. F. BursLey TAyLor. 
Fort Wayne, Ind., April 9, 1892. 
A Correction. Note on the Paper on Devonian Rocks of Buchanan 
County, Towa, in the Americun Geologist for September, 1891—In the paper 
mentioned in the title of this note I made the statement that beds of 
brecciated limestone lie at the base of the Devonian series in Buchanan 
county, Iowa, and that this was succeeded by the Independence shales. 
The Independence shales were originally exposed by putting downa 
shaft in the bottom of what is known as the old Kilduff quarry, near In- 
dependence. In this shaft, beds containing Gyroceras and Gypidula were 
found immediately overlying the shales, but none of the beds showed 
any signs of being brecciated. Beds of brecciated limestone are ex- 
posed in the bed of the river below the bridge at Independence, and it 
was concluded that since no such beds were found above the shales at 
Kilduff’s, the breccia must lie below them. Subsequent observations, 
made more thoroughly and in detail, revealed this state of affairs: The 
Gyroceras beds to a thickness of twenty feet or more, have in general, 
over an area reaching at least from Fayette to Troy Mills, been broken 
up, and the angular fragments re-cemented to form the brecciated lime- 
stone so conspicuous at many widely separated points within the Devon- 
ian area of Iowa. The beds affected were not entirely converted into 
breccia, for it is occasionally found to be the case that over patches sev- 
eral rods in diameter the Gyroceras beds, with their associated layers, 
retain their original position undisturbed. It would seem that the shaft 
at the Kilduff quarry passed through one of these undisturbed patches, 
and so led to the conclusion that the breccia exposed at the bridge in 
Independence could not be above, but must lie beneath, the shales. Those 
who are interested will kindly note this correction in connection with 
the article in the Gronoarst, Vol. VIII, p. 142. 
A few points have been found where the shales, by a little digging, 
may be seen beneath the breccia. S. CALVIN. 
