220 SPARKS FROM A GEOLOGIST'S HAMMER. 



Another effect of the perturbating influence of the lakes, 

 reacting upon topographical and continental relations, is 

 to cause certain isothermals to divide and, by reuniting, 

 to inclose detached areas, which stand like islands of cold 

 or heat. An example of the former exists in the penin- 

 sula of Michigan, and one of the latter in Iowa. The 

 greater part of Ohio, however, seems to constitute an 

 island of uniform temperature in July, since from Cleve- 

 land to Marietta and Portsmouth, the mean is not far 

 from 73i. 



The distribution of the January isothermals possesses 

 still greater interest. It is the severity of our winter 

 climate rather than the character of summer which, in 

 our northern states, conditions the growth and health of 

 most of our perennial exotics, as peaches, apples and im- 

 proved varieties of grapes. With a glance at the chart 

 of January isothermals, the eye is first arrested by the 

 general northward deflection of the lines in the vicinity 

 of Lakes Michigan and Huron. This direction is the re- 

 verse of the July inflection. The isothermal of 23, for 

 instance, which passes through Peoria, Illinois, enters the 

 southern extremity of Lake Michigan, and proceeds di- 

 rectly to Northport, at the mouth of Grand Traverse 

 Bay. It thence sweeps southward to Lansing, when it 

 returns northward, under the influence of Lake Huron, to 

 Thunder Bay Island and finally bends eastward, passing 

 forty miles south of Penetanguishene in Canada. 



Similarly, the isotherm of 27 sweeps from southwest- 

 ern Michigan through Springfield, Illinois, and thence to 

 Fort Riley, in Kansas, near the latitude of 39. East- 

 ward, the same isotherm strikes through Central Indiana 

 and Ohio. The January climate of New Buffalo is as 



