GRAIN OF ROCKS. 403 



The following paper was then read : 



GRAIN OF ROCKS 

 BY ALFRED C. LANE 



[Abstract*] 



Contents 



Page 



Introduction 4°3 



Grain as affected by rate of cooling 403 



Verifications 4°5 



Mathematical equations 4°6 



Introduction 



The slower crystals form, the more likely they are to be coarse. If we wish to 

 produce a glassy candy, like barley candy, the syrup should be cooled as quickly 

 as possible. Among rocks we have the glassy lava of obsidian and the coarsely 

 crystalline granite of the same composition. In the case of obsidian, the difference 

 is generally associated with more rapid consolidation. More precisely, we may say 

 that whether a solid be formed from liquid, as beds of salt are thrown down from 

 the sea, or formed by loss of heat, as in the chilling of lava, or from the escape of 

 gas, as when C S % escapes from tire cement, the more rapid the solidification when 

 crystallization results, the smaller are the constituent crystalline grains. 



Grain as affected by Rate of Cooling 



Singling out one particular manner of solidification for attention, namely, that 

 of solidification by cooling, let us at first confine our attention to the case of a sheet 

 so extensive with regard to its thickness that we need only regard its cooling in 

 the direction of its thickness. Suppose it to have a uniform initial temperature 

 and constant conductivity and its surfaces kept at a constantly fixed lower tempera- 

 ture while it cools, such will roughly be the conditions of a dike suddenly intruded 

 into wet rocks. 



As it cools, the temperature close to the surfaces will fall very suddenly nearly to 

 that of the fixed temperature of the margin. Toward the center the cooling will 

 be less rapid. In fact, there will be a short space of time, increasing toward the 

 center, during which the cooling effect of the lower temperature at the margins 

 will be hardly felt. Then the cooling will increase and as the temperature drops 

 to near that of the margin grow less rapid once more. 



These facts are illustrated by figure 1, which is a sketch from a plate showing the 

 cooling of an igneous sheet 100 feet thick and of an indefinite extent. The value 

 400, as given in Thomson and Tait's Natural Philosophy (page 476), is used for the 

 conductivity, and temperature curves are given not only for the center, but for 

 points at various distances from the center toward the margin as indicated. These 



*The mathematical treatment of the subject and a plate illustrating the solution, the applica- 

 tion to Keweenawan ophites, and the chemical relations are reserved for fuller treatment in 

 volume vi of the Michigan Geological Survey reports. The experimental illustrations will not be 

 found there. 



