288 The American Geologist. November, 1894 
wards, N. Y. ; and again in the valley of the Missisquoi river 
in northern Vermont. In all these places this deposit presents 
not only the same general appearance and relation to other 
contiguous deposits, but also a remarkable uniformity of finer 
details. Take, for example, the silt beds at liracebridge. 
The whole set of phenomena at this place is extremely in- 
structive. The laminations of clay and silt are associated in 
pairs which are almost without exception about half an inch 
in thickness. On weathered surfaces the principal part of 
each layer is a greenish gray clay, and this is separated from 
the next layer of clay in each case by a layer of white silt, an 
eighth to a sixteenth of an inch in thickness. There are some 
variations in the composition of the deposit at each locality, 
but they are confined chiefly to varying proportions of the 
two materials. In a few places I found the clay almost ab- 
sent and the silt layer thicker than usual. In other places 
the variation was reverse of this. It seems plain enough that 
the silt and the clay must represent two slightly different 
conditions of sedimentation ; and the orderly way in which 
the layers alternate shows that a layer of silt and a layer of 
clay taken together 3onstitute one complete round of change. 
This points to recurrence and almost certainly to periodicity. 
Tides, storms, and the annual round of the seasons, are the 
only recurrent variations liable to affect sedimentation. Of 
these the tides and the seasons are periodic, but storms are 
irregular. Neither tides nor storms afford a satisfactory ex- 
planation. For the one is much too short in its period, and 
the other too irregular. It seems impossible that the pairs of 
layers can represent anything but annual periods of deposi- 
tion, and if this be the case several important conclusions 
follow. Considering the great thickness of the whole deposit, 
the length of time which must be allowed for its formation 
can hardly be less than several thousand years. Indeed, if 
we suppose the laminations to be uniform, and the maximum 
depth of the whole original deposit to have been 100 feet, the 
time of deposition would be about 2,500 years. And this, it 
should be noted, would be not the whole time of the submer- 
gence, but only the time during which the conditions of still- 
water sedimentation existed at that level, not counting the 
two periods unfavorable to this kind of sedimentation, one as 
