OF THE NORTHWESTERN STATES. 31 



Potsdam up to the Medina sandstone, embracing the Utica slate, Shawangmik grit, 

 and Hudson river group ; beds not calculated to resist denuding forces of any kind. 

 Lake Champlain formed a lateral channel also on the line of strike. 



Probably the series of interior lakes of Middle New York, with their axes north 

 and south, will on examination show a local parallelism with the drift force. Pass- 

 ing to Lake Erie, there is less uniformity. The course of the striae along the south 

 shore A'aries from south to south 80° east. 



Of their bearing on Lake Huron I have no information ; but what I have seen 

 on Lake Erie indicates a meeting of forces in this neighborhood. The strata at 

 the bottom of this lake are such as to be easily reduced ; but the lake is a shallow 

 one, nowhere reaching 300 feet in depth. Most of it is less than 150 feet. 



The upper silurian limestones form the shore at both ends, curving to the north 

 into Canada. Above these rocks are the Hamilton and MarceUus slates, more 

 properly shales, which are soft and clayey. These shales form the southern coast 

 from Cattaraugus creek to Huron river, and outcrop beneath the water towards the 

 Canada shore. If there had been a steady movement from northeast to southwest, 

 along the edges of these strata, there should have resulted a deeper depression. 

 The same shales extend through the greater part of Lake Huron, dipping to the 

 west and southwest. 



Indications are that the movement was here as on Lake Erie, across the strike 

 of the beds, or, if not so, it was irregular. In the Straits of Mackinaw it was from 

 east to west. There is in Central Michigan no high land to divert the glacier 

 mass from its general southwesterly course ; and the arm of Lake Huron, known 

 as Saginaw bay, approaches very near to the central portion. 



It is probable that this bay will be found to be one of the channels which it fol- 

 lowed. The western outcrops of the same soft shales and sandstones lie beneath 

 Lake Michigan. Around them are the same upper silurian limestone beds that are 

 seen on the north shore of Lake Huron. The western coast of Lake Michigan is 

 composed of these silurian strata, which have much more resistance than the shales. 

 Along the western shore of this lake the glacial striae have a very uniform direction, 

 which varies little from southwest. 



Along the north shore their bearing is more Avesterly. There was a deflection 

 to the southward along the axes of Lake Michigan, Green bay, and Lake Winne- 

 bago, which brought the motion nearly into parallelism with the outcrop of the 

 rocks. 



The fac-simile of striae at Sheboygan shows two sets of lines, one more to the 

 east than the other. That curve corresponded very closely with the change of dip 

 and bearing of the rocks. The lower peninsula of Michigan occupies the centre of 

 a geological basin, the surface rocks of which are the coal series. The rocks of 

 the upper peninsula of Canada, northern Ohio, Indiana, and Wisconsin dip beneath 

 this coal series on all sides. 



On the north shore of Huron and Michigan, the inclination is to the south. On 

 the western shore of Michigan it is southeasterly, and at length eastwardly, having 

 the slates of the Hamilton and MarceUus groups on the east and at the bottom of 



