328 J. P. Iddings — Columnar structure in the 



rapidly than the other they will no longer curve to the same 

 extent and the slower one will curve more than the faster one. 

 Instead of two single columns there are always two groups 

 approaching one another, fig. 5, and these prevent the continua- 

 tion of the columns beyond the curve, pinching them out and 

 causing them to taper off as already observed in the quarry 

 described. 



The difference in the systems of cracks of the lower and 

 upper portions of this lava sheet may then be accounted for by 

 a difference in the rate of cooling from the bottom and top 

 surfaces. The more frequent fractures arising from the more 

 rapid cooling ; the two systems proceeding from their initial 

 planes until they blend in one another within the mass. If for 

 any reason the cooling from one surface should take place 

 irregularly and from any point proceed more rapidly than from 

 others, it is evident that there would result a set of curving 

 columns diverging from this point as a focus. The curving 

 and diverging of the upper sets of columns in this locality then 

 may be ascribed to differences in the rates of cooling from the 

 upper surface, which might be brought about in a variety of 

 ways. What were the most likely ones in this instance it is 

 difficult to say, since erosion has carried away the top of the 

 layer at this place and left us in doubt as to its original condi- 

 tion. 



There remains to be considered the subsequent contraction 

 exerted in all directions through the prisms into which the 

 mass has been divided, for the effect of surface cooling having 

 been expended in producing the prismatic cracks, the subse- 

 quent contraction results from a still more gradual cooling of 

 each of these portions with little reference to the source of 

 cooling. The uniform contraction of a homogeneous body acts 

 equally in every direction through it, and its effect corresponds 

 to the equal shortening of the radii of a sphere of such a body. 

 If from a uniform resistance cracking or parting occurs it will 

 take the form of concentric spherical shells. If for any reason 

 the uniform resistance to the contracting force be counteracted 

 by some other force acting in a particular direction the parting 

 will no longer be spherical but ellipsoidal, as will be seen from 

 fig. 6, where abc represents a section through the sphere along 

 the radii of which contraction takes place. A uniform resist- 

 ance in the direction of the radii represented by cc', dd\ etc., 

 will produce a parting parallel to the arc of the circle c'd'e', etc. 

 If, however, the resistance in a direction parallel to the line ac 

 be neutralized by some force, the resistance along the different 

 radii will be diminished by the amount of the component par- 

 allel to ac in each case, and the resulting fracture will be 

 parallel to the ellipse cb\ the relative tendency to fracture also 

 being indicated by the area bcb'. 



