104 GEOLOGY OF EASTERN WISCONSIN. 



channel of water communication between the Mississippi and the 

 northern lakes. Important facts developed by that survey may be 

 found in Ex. Doc. No. 15, of the House of Eepresentatives, 40th 

 Congress, and should not be forgotten by an enterprising people. 



Eeturuing from this digression, it is to be remarked that the diag- 

 onal valley previously described, and this Rock river, Green Bay val- 

 ley, unite at the north and join the great depression occupied by Lahii 

 Michigan. The bed of this great lake is excavated chiefly from the 

 soft rocks of the Devonian age, and has its axis parallel to the strike 

 of the formations. The western edge of .the lake rests upon the jS")- 

 agara dolomites, but the dip of the formation is greater than the 

 slope of the lake bed, so that this formation is probably overlaid near 

 the western edge of the lake by the upper formations. 



This great submerged valley possesses one of the main features 

 that characterize the Green Bay valley, that of having a more abrupt 

 slope on the east. At least this is true of the northern portion accord- 

 ing to the lake survey charts, for which I am indebted to the kindness 

 of Gen. C. B. Comstock. This, however, is not a conspicuous fact, 

 and the eastern shore is far from being precipitous. The eastern line 

 of Wisconsin would, if traced on the bottom of Lake Michigan, lie 

 almost wholly helow the sea level. The extent to which Lake Michi- 

 gan occupies the territory of the state and its nearly constant level, 

 make its surface our most convenient datum plane in giving elevations 

 and discussing topographical features. The elevation of the surface 

 of the lake above the sea level, that has been adopted in the progress 

 of the survey, is 578 feet. In the report of the survey of Kock river, 

 576 feet is given, and Gen. Comstock informs me that the lake survey 

 use at present 581 feet above mean tide at New York. The series of 

 levels, now being run, will, when completed, give for the first time an 

 accurate determination of the level of the great lakes. 



The erosion of the great valleys we have now considered left of 

 necessity adjacent slopes and dividing ridges. North of the Fox 

 river a very large area is included in a southerly and southeasertly 

 incline, the drainage of which is tributary to that river. This is indi- 

 cated clearly by the general course of the "Wolf, Oconto, Peshtigo and 

 Menoraonee rivers and their branches. On the west side of Eock 

 river there is a similar slope tributary to it. But the most conspicu- 

 ous elevation in the district is the extensive ridge that lies immedi- 

 ately east of the valleys of Green Bay and Eock river, and which has 

 already been cursorily mentioned in the descriptions of those valleys. 



A glance at the course of the rivers of this region shows that the 

 summit of this elevation is immediately adjacent to the valleys men- 



