222 SPARKS FKOM A GEOLOGIST'S HAMMER. 



felt across the entire peninsula. The mean of Flint, for 

 instance, is four degrees above that of Prairie du Chien, 

 on the same parallel. The narrowing of the peninsula 

 northward emphasizes this consideration. Secondly, Lake 

 Huron exerts its proper influence upon the western 

 shore, which reinforces that brought from Lake Michigan. 

 Thirdly, the intrusion of Saginaw Bay into the interior 

 throws a large area to the east and southeast of this 

 body of water. It may also be mentioned that the po- 

 sition of this bay, and the peculiar bend of Lake Huron 

 toward the west, are such that even north winds must 

 come somewhat tempered by these great natural stoves- 

 It is certainly a singular circumstance that, while Mani- 

 towoc, Milwaukee and Chicago, on the west shore of 

 Lake Michigan, have lake-winds during January, repre- 

 sented by the numbers 11, 18 and 7 respectively, Thun- 

 der Bay Island, Ottawa Point and Fort Gratiot, on the 

 west shore of Lake Huron, have winds from that lake 

 during January, represented by the numbers 51, 86 and 

 35. These numbers embrace north winds at Chicago and 

 the points on Lake Huron, and southwest winds at Ot- 

 tawa Point, as these sweep along the axis of Saginaw 

 Bay. 



The isothermal chart of the lake region for January 

 exhibits in the country south and southwest of Lake Su- 

 perior a series of remarkable loops. The great isotherm 

 of 14, for instance, coming down past the head-waters of 

 the Minnesota river, passing near Saint Paul, and con- 

 tinuing southeastward to the 44th pai-allel, begins to feel 

 the influence of Lake Michigan, and bends northeast 

 through the region west of Green Bay to the narrow 

 peninsula north of Lake Michigan, where, under tie in- 



