48 | GEOLOGY OF OHIO. 
the strize entered the water. The rocks immediately next to the lake 
here are too soft to retain strie, but in going back two or three miles to 
the elevated escarpment of hmestone four hundred feet above the lake, 
the rock surface is seen to be covered with strie, running in nearly the 
same directicn as at the easterly end, or 8. 35° W. Standing on the edge 
of this escarpment and looking towards the north-east, whence the ice 
eame, it can be seen at a glance that it must have crossed the basin of 
the lake; and still further to complete the proof, in the bed of Till on the 
summit of the escarpment there are plenty of striated fragments of the 
. Cambro-Silurian strata, (Hudson River) which, from the course of the 
strie, must have been brought from the outcrop of these beds in the bot- 
tom of the lake. When the path of the glacier can be thus traced fol- 
lowing the-axis of the lake from the north-east to south-west, and masses 
of Till which have been eroded from the rocks outcropping in the area of 
the lake are met with, heaped up on its banks at the south-westerly end, 
the only conclusion which can be drawn is that the lake basin is due to 
the powerful eroding influence of a glacier.” 
Dr. Edmund Andrews, of Chicago, who has studied with much care and 
success the surface geology of the country bordering Lake Michigan, in 
his paper on The North American Lakes considered as Chronometers of Post- 
Glacial Time (Trans. Chicago Acad. Sci., Vol. I), attempts to estimate the 
time which has passed since the close of the Drift period, by measuring 
the erosion of the Drift clays by the shore waves, and the accumulation 
of sand—the result of such erosion—about the head of Lake Michigan. 
This paper is an interesting one, and is the product of much careful and 
conscientious study, but it can only be considered as furnishing data for 
measuring approximately the time during which the lake has stood at 
its present level. This is, however, only the last chapter in the history 
of Post-Glacial events, and it may represent but a fraction of the time 
which has elapsed since the glaciers retired from the lake basin. Of all 
the earlier period, when the water of the lakes stood several hundred 
feet higher than now, and when the upper beaches of Lake Superior, 
Lake Huron, Lake Erie, and Lake Ontario were formed, no record remains 
in the vicinity of Chicago, as the shores of Lake Michigan are so low 
that when the waters of the lakes was highest, they were deeply sub- 
merged. 
Prof. James Geikie devotes a chapter in his “Great Ice Age” to the 
Drift Deposits of North America. In this he labors under the serious 
disadvantage of never having seen with his own eyes the phenomena he 
describes. He is compelled, therefore, to trust to the testimony of widely 
separated observers, studying different phenomena, and very unequally 
