So The American Geologist. A "" ,|M - L903 - 
the area above outlined to form a trough or catch pit, whose 
greatest depth was in middle Pennsylvania and which shoaled 
from that point in all directions. The greatest elongation of 
this trough, or of so much of it as now remains, is along a line 
from N.N.E. to S.S.W., hut its E.-W. extension has heen so 
much obscured by subsequent folding and erosion that its orig- 
inal dimensions are indeterminable and not improbably its di- 
ameters in both directions were approximately equal. 
Into this basin, or trough, were poured the water and the 
sediment brought down by the rivers from the contemporary 
Atlantic land. Subsidence and filling went on equally and from 
the nature of the deposits can be gained some knowledge of 
the changes undergone by the eastern shore of the gulf, which 
was undoubtedly the quarry from which the material was ex- 
cavated. 
These deposits contain, in the extreme northeast on the 
Hudson, a considerable proportion of sand as shown in the 
flagstone quarries near Kingston, etc. Hence the elevation 
closing the St. Lawrence channel was still proceeding and- the 
consequent erosion was sufficient to bring off the new land the 
coarse sediment there characterizing the Hamilton strata. New 
England was still rising and increasing and its harder strata 
were exposed to the weather. At the same time the small thick- 
ness and extent of the flagstones show plainly that the move- 
ment to which they were due was not excessive or rapid, allow- 
ance being made for the subsequent erosion of a great part of 
them. The bulk of the Hamilton beds even here consists of 
soft material more or less calcareous. The only geographical 
change indicated by the geology is a progressive enlargement of 
the northeastern land and a partial filling of the gulf in Xew 
York. 
Farther south are indications of a movement which, though 
also local, yet produced a marked change in the sedimentation. 
The Hamilton in middle Pennsylvania contains a massive sand- 
stone, the first that we find in the column above the more mas- 
sive Medina of early Silurian date. The ( h'iskany is thin. It 
underlies Perry, Dauphin, Juniata, Mifflin, Snyder. Union, 
'Montour, Columbia and Huntingdon counties in whole or in 
part, with some adjacent portions of others, and may be rough- 
ly estimated to have an area of at least 4000 square miles. It 
