Mesozoic Igneous rock of New Jersey. 325 



the surface, under these conditions, will be least in a direction 

 parallel to the surface and greatest in one normal to it, and 

 when the limit of tension is reached the resulting rupture 

 which takes place at right angles to the direction of maximum 

 strain, will be parallel to the surface of the mass. Aa the con- 

 solidation due to surface cooling proceeds inward the resistance 

 to contraction parallel to the surface increases at a greater rate 

 than that normal to it, a point may then be reached where re- 

 sistance in the first named direction will exceed that in the sec- 

 ond and the resulting rupture will be perpendicular to the 

 cooling surface. 



Hence it so frequently happens that the rapidly cooled, 

 upper portion of a surface flow of lava is divided into plates 

 approximately parallel to its surface while the more slowly 

 cooled mass beneath is cracked more or less vertically. For 

 the same reason also the quickly cooled sides of an intrusive 

 mass are split into plates by cracks parallel to the contact 

 surface, the cracks being closest together nearest the surface, 

 where the lateral resistance to contraction has been least. 

 These cases of tabular parting, however, must not be con- 

 founded with the general tabular parting of many lava flows 

 which arises from inherent lamination due to layers of different 

 texture or consistency, producing planes of weakness through 

 the mass. 



Columnar structure then is found in that part of a lava flow 

 which has consolidated and contracted slowly under the influ- 

 ence of surface cooling, and the laws governing its develop- 

 ment may be reached by considering the origin and progress of 

 a crack caused by the shrinkage of a homogeneous mass through 

 surface cooling. Starting with a plane surface over which 

 forces producing contraction are acting uniformly, the contrac- 

 tion produced on the surface of the mass in a given time will 

 be greater than that produced at some depth within the mass, 

 and will decrease gradually inward. As the contraction pro- 

 gresses the limit of tension in the direction of the surface will 

 be reached before that in the direction of depth causing a rup- 

 ture across the direction of the surface, and as the limit of 

 tension for the layer next below is reached it will rupture in 

 the same direction as the surface layer did, and so on. The 

 direction of the crack will be at right angles to that of greatest 

 contraction, or normal to the line of maximum strain. The 

 condition of the mass at the moment the limit of tension along 

 the surface is reached may be graphically represented as in fig. 

 1, the contraction being a maximum in the top layer and 

 diminishing successively in each layer beneath to that with 

 the initial expansion ; the distance of this layer from the sur- 

 face being taken as unity the maximum contraction at the 



