Mesozoic Igneous rock of New Jersey. 327 



over the surface ab, fig. 2, being a maximum at c, the maxi- 

 mum strain at the beginning will be in the direction of the 

 surface and the cracks will start normal to it, but their progress 

 inward will no longer be uniform. At the end of a given time 

 the limit of tension reached bj a greater force, at f, will be 

 farther from the surface than that reached by a less force, at g, 

 and the line of maximum strain in this portion of the mass will 

 be g' ' f ', to which the crack of parting will be normal. At the 

 end of another given time the direction of the crack will be 

 again changed, and the same action taking place in other parts 

 of the mass will result in a system of diverging and curving 

 cracks. 



If the surface from which uniform cooling takes place is not 

 plane but warped it is evident that the cracks normal to it will 

 start in various directions through the mass. 



So far we have considered the shrinkage in one plane only, 

 that is in a plane at right angles to the cooling surface. But 

 in a homogeneous mass the contractile force which produces 

 cracks at certain distances will exert itself equally in all direc- 

 tions over a surface uniformly subjected to the cooling forces, 

 and will at the instant of rupture act toward centers whose 

 distance apart is dependent on the rate of cooling. If the mass 

 is perfectly homogeneous the centers of contraction will be 

 disposed over the surface with the greatest uniformity possible, 

 that is, they will be equidistant throughout, fig. 3, and the re- 

 sultant fractures will be in a system of hexagons; the inward 

 progress of these divides the mass into prisms. If from any 

 irregularity in the composition or petrographic structure of a 

 rock the contractile force acts unequally in different directions. 

 The form of the polygons will be less regular. Those most 

 commonly found in rocks are quite irregular, adjacent prisms 

 having four, five and six sides of unequal size. 



The mutual influence of the forces producing different 

 prisms as they approach each other is readily understood from 

 the following: take the case of two columns s, s', fig. 4, ap- 

 proaching one another, and suppose the progress of the max- 

 imum strain to have reached ab, a'b' ', the forces producing con- 

 traction acting through a and a' will meet and react on each 

 other before those acting through b and b ', so that the points of 

 maximum strain at the end of a given time will have advanced 

 farther along the lines through a and a' than through b and V . 

 The lines of greatest strain will then be cd and c'd' and the 

 cracks normal to them will take the direction ce and cV. This 

 reaction will continue till they become parallel. 



If there were but two columns forming at equal rates they 

 would curve symmetrically and continue in parallel directions 

 and of constant width, but if one column progresses more 



