PRISMATIC STRUCTURE IN IGNEOUS ROCKS 217 



in previous discussions. If the mass is cooling slowly, the crystal- 

 lized shell may be able to adjust itself by a slow movement to the 

 stress produced by contraction, so that the strain does not for some 

 time pass a given value. If the cooling is rapid, on the other hand, 

 the strain may be rapidly raised through the inability of the mass 

 to flow as rapidly as the stress is applied. Under conditions of 

 rapid cooling, therefore, the temperature at which the stresses 

 become sufficient to produce rupture will be higher than under 

 conditions of slow cooling. 1 



- Another point concerns the conditions of rupture. Published 

 discussions of formation of columns by contraction have tacitly 

 assumed that the condition of rupture is that the extension shall 

 exceed a certain limiting value. This is only one out of several 

 possible conditions of rupture. Various hypotheses have been pro- 

 posed by physicists (limiting tension, limiting positive or negative 

 strain, limiting shear), of which the best founded experimentally is 

 that of Tresca and Darwin, according to which rupture occurs when 

 the maximum difference of the greatest and least principal stresses 

 reaches a certain limiting value. 2 Although the acceptance of this 

 condition of rupture as the fundamental one does not simplify the 

 problem of calculating the actual physical magnitudes of tempera- 

 ture, temperature gradient, stress, and strain in any given case, yet 

 it should permit a more complete analysis of the kinds of structure 

 that will result from different conditions of cooling. Such an 

 analysis is, however, beyond the scope of the present article. 



CONTRACTION OF PHYSICALLY HETEROGENEOUS MATERIAL 



Prismatic structure is very common in materials which are 

 heterogeneous as regards their state of aggregation (such as mud 

 and wet starch), that is, in which solid matter is suspended in or 

 mixed with a liquid. It is a question whether the formation of a 

 prismatic structure in such materials is strictly comparable with 

 most cases of contraction prisms in igneous rocks. The principal 



1 The above-mentioned effect of the rate of cooling is quite distinct from the 

 commonly recognized effect, which appears in the temperature gradient away from the 

 surface of the cooling mass. 



2 Love, Theory of Elasticity, 1906, p. 119. 



