28 Messrs. Foster and Whitney on the 
around the northern shores of lakes Huron and Michigan. From 
this point, they increase in thickness as traced to the east and to ' 
the west. On the other hand, the lower limestories—the Chazy, 
Birds-eye, Black river, and Trenton groups—appear to decrease 
gradually in thickness, as traced westwardly from the Mohawk 
valley, and the outlet of Lake Ontario, where they exist in great 
force. This fact is made very conspicuous, when their entire 
thickness, as developed in New York, is compared with that of 
the same groups on the northern side of Lakes Huron and 
Michigan, where they contract to within a hundred feet, and in 
some places even less. We have also evidence that they do not, 
like the preceding groups, inerease in thickness, when traced into 
Wisconsin and across the Mississippi; for there, they hardly 
attain a vertical range of fifty feet. Although over the entire 
area the identity of character and the continuity of the beds are 
maintained, it is clear that the materials have continually under- 
gone diminution, and that the formation, unless it has been 
supplied from other sources, will be found to die out. 
“In t 
undergone when traced over a considerable area. In the district 
under consideration, it is clearly recognized throughout its whole 
extent, from Drummond’s Island to Green Bay, except where it 
has been denuded, and the space occupied by lakes or bays. 
This group, like the other members of the limestone series before 
alluded to, exhibits a diminution in its thickness, as traced west- 
wardly, and disappears, a short distance beyond the limits of the 
district. 
reverse is true. Leaving out of view the Medina sandstone and 
the arenaceous portions of the Clinton group, which are scarcely 
recognizable here, we find the succeeding limestones muc 
increased in thickness, and exhibiting no diminution, but rather 
an augmentation, as traced to the westward. 
“From these general remarks, the reader will be prepared to 
understand the details given under each division of the series. 
It should be observed, however, in the outset, that the width of 
surface occupied on the map, by a particular group, is not always 
to be regarded as an indication of its thickness; for this, in most 
ases, is dependent upon the amount of dip in the beds, which, 
in this case, nearly corresponds with the slope of the country ; 
so that it often happens that a group, less than a hundred feet in 
thickness, forms a belt several miles in width. ‘ 
“Th ring and inclination of these successive groups indi- 
cate that they formed the outer margin of a great geological basi, 
= Sem a er ee oem 
