698 DYNAMICAL GEOLOGY. 



a minimum, 560,000 years ; an extreme maximum, 610, 000 years, and so on. The points 

 of maxima and minima are repeated approximately at intervals of 1,450,000 years. 



At a time of maximum eccentricity, while almost precisely the same 

 amount of heat, as now, would reach the earth, it would be differently 

 distributed through the seasons. If, during an era of great eccentri- 

 city, the winter of the northern hemisphere occurs in perihelion and 

 the summer in aphelion, the winters will be mild and the summers 

 cool ; and the reverse will be true for the same era in the southern 

 hemisphere, the winter there occurring in aphelion, and cold, and the 

 summer in perihelion, and hot. Geological effects, from this condition, 

 would be manifested in one hemisphere, according to Croll, in the 

 snows of winter continuing long into the summer, or through it, and 

 even, as he argues, in bringing on the conditions of a glacial era. 

 But, while this was the condition in one hemisphere, the other would 

 have an equable climate. Croli states that the glaciated hemisphere 

 would be that which had the cold winter ; while J. J. Murray holds * 

 that it would be the one that had the cold summer. Professor H. A. 

 Newton (to whom the author has submitted the subject) believes it 

 very questionable whether the differences of eccentricity, and the con- 

 sequent differences of summer and winter temperatures, are capable 

 of producing so great a result in either direction. 



2. Chemical and Mechanical Action. — Heat is evolved by chemical 

 changes in which there is condensation, as in liquids becoming solids, 

 or gases, liquids, etc. ; often an effect of the natural decomposition of 

 minerals, or of vegetable or animal matter. 



Mechanical action, as the beating of waves on a coast, the falling of 

 water in cascades or rain, the shakings of earthquakes, sliding of rocks, 

 motion of the atmosphere in winds, produces heat, whenever the action 

 meets with resistance, on the principle that motion corresponds to an 

 amount of heat, or that heat is transformed motion. The heat thus 

 resulting is, however, of very little geological importance. But the 

 motion attending uplifting, plicating, shoving, along fractures, and crush- 

 ing of rocks, is, as demonstrated by Mallet, an efficient and wide-reach- 

 ing source of heat and of geological work ; for heat thus originated 

 among the earth's strata has been an important means of consolida- 

 tion, metamorphism, and probably even of fusion. 



Mallet demonstrates, by many careful experiments, that the crushing of a cubic foot 

 of syenyte or granj'te produces 119 to 213^ degrees Fahrenheit; of two slates, 132-85 

 and 144*29 degrees; of three sandstones, 32-84, 47-79, 86-13 degrees; of two compact 

 limestones, 20-98, 26.28 degrees; of Devonshire marble, 114-68 degrees. He obtained 

 for the specilic heats of the same rocks, the syenytes andgranytes, 181 to 0-196; slates, 

 0-201, 0-218; sandstones, 0-238, 0-233, 0-215;" limestone, 0-245, 0-265; marble, 203. 2 



1 Quarterly Journal of the Geological Society, xxv. 350, 1869. 



2 Mallet on Volcanic Energy, Trans. Roy. Soc, 1872. 



