246 The American Geologist. October, 1894 
eial erosion and deposition will be represented by about eight 
square miles. 
During the geologically short Postglacial or Recent period, 
which has been shown by the independent but well agreeing 
observations and measurements of N. H. Winchell, Gilbert, 
Andrews. Wright, and many others, to have comprised only 
some 6,000 to 10,000 years, the material eroded here has been 
again undergoing transportation to the sea or to the Manitoba 
lakes or lower portions of the great avenues of drainage. In 
this period the amount eroded from the surface along these 
800 miles probably has not averaged more than two or three 
feet, its total section therefore being represented by about 
two-fifths of a square mile. 
These approximate measures of the denudation along the 
international boundary during the successive divisions of time 
since the Cretaceous period seem consistent with the estimates 
presented in my former paper, that the duration of the Terti- 
ary era was probably between two and four million years; of 
the Lafayette period, between 60,000 and 120,000 years ; of the 
Glacial period, perhaps 20,000 to 80,000 years; and of the 
Recent period, between 6,000 and 10,000 years. 
INTERRUPTION DURING THE DEPOSITION OF 
THE BURLINGTON LIMESTONES. 
By Francis M. Filtz, Burlington. Iowh. 
The lithological characters of the different strata of the 
Burlington limestones are quite generally the same. Even the 
upper division does not differ materially from the lower. Of 
course some of the layers are more compact than others, a few 
even being massive and crystalline enough to resemble mar- 
ble. Yet whether massive or not they have a common origin. 
They are all crinoidal. With the exception of a very few lay- 
ers you could scarcely find a cubic inch of rock that does not 
plainly show its crinoidal origin. Between the beds of lime- 
stone are found a few layers of shales, clays, etc., but they 
form but a comparatively small part of the whole. 
It has generally been conceded that the deposition of the 
whole Lower Carboniferous group in southeastern Iowa was 
