218 SPARKS PROM A GEOLOGIST'S HAMMER. 



tation of a mean, the more the salient features of the 

 climate are disguised. The annual mean in the Lake 

 Region approximates that of other districts in the same 

 latitude, since the cooling effect of the lakes in sum- 

 mer is neutralized in the annual mean by the warming 

 effect in winter. We approach nearer an expression of 

 the local peculiarities of the climate by comparing, as we 

 have done, the seasonal means. But we approximate still 

 nearer an exhibit of the special climatic conditions by 

 making comparisons of monthly means especially for 

 those months whose temperatures depart most from the 

 annual mean, and from the mean temperature of the 

 lake water. These months are July and January. 



If we inspect the isothermal chart for July we shall 

 observe a series of lines drawn through localities of equal 

 mean temperatures, within the limits of the region affected 

 by lake influence, and extending far enough westward to 

 reach the general continental conditions. The first thing 

 which impresses one is the extreme southward deflection 

 of all the lines in the vicinity of Lake Michigan, and a 

 similar, though less abrupt, deflection in the vicinity of 

 Lake Huron. Tracing, for instance, the line of 70, we 

 find it entering from the west on the parallel of 48. Its 

 course is southeast, under the influence of continental 

 conditions, as far as Fort Ripley, in Minnesota, whence 

 it passes nearly eastward to the valley of the Menominee 

 River. Here it comes under the decided influence of 

 Lake Michigan, and rapidly bends southward, passing 

 through Green Bay and Milwaukee, in Wisconsin. Re- 

 appearing at Grand Haven, in the peninsula of Michigan, 

 it trends almost directly northward to Traverse City, 

 whence it arches across the peninsula till, coming within 



