48 GEOLOGY OP OHIO. 



the strise entered the water. The rocks immediately next to the lake 

 here are too soft to retain strife, but in going back two or three miles to 

 the elevated escarpment of limestone four hundred feet above the lake, 

 the rock surface is seen to be covered with strise, running in nearly the 

 same directi-rn as at the easterly end, or S. 35° W. Standing on the edge 

 of this escarpment and looking towards the north-east, whence the ice 

 came, 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 j)lenty of striated fragments of the 

 Cambro-Silurian strata, (Hudson River) which, from the course of the 

 striae, 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 Brie, 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 



