production of the Prismatic Structure of Basalt, 207 



development of their differently shaped crystals. The entire 

 mass, forced up through fissures from below and overflowing 

 horizontally, assumes the interior conditions of a stream or 

 streams of imperfectly liquid matter, in which the floating crys- 

 tals arrange themselves with their broader sides more or less 

 horizontally, and their greater lengths in the direction of the 

 flow. A distinct grain is thus often impressed upon the entire 

 mass, tending to produce differences in the facility of fracture in 

 two vertical orthogonal planes and one horizontal plane, the latter 

 being the one of easiest fracture in the mass. From the fine- 

 ness of the particles of basalt and the want of large imbedded 

 crystals, these characteristics are but ill pronounced and are fre- 

 quently not discernible ; but they only differ in degree from the 

 well-known structure, likewise developed by flow, which has been 

 remarked, and is well known to the quarry-men in the Dartmoor 

 and many other granites, which possess three orthogonal planes 

 presenting differences of facility of fracture. The horizontal one is 

 that of easiest fracture * and in it are found the flat surfaces (gene- 

 rally) of the large separate crystals of felspar, and many of those of 

 black mica also. The plane of next easiest fracture is a vertical 

 one, which generally coincides with the longest axes of these 

 crystals ; while the plane of most difficult fracture is a vertical 

 one orthogonal to the last, and therefore transverse to the line 

 of flow of the granite when molten. In addition to this struc- 

 ture, more or less developed, the lower part, which has received 

 the greatest pressure from the superincumbent mass of the basalt 

 itself and of matter heaped upon it, must be denser than the 

 higher portions. All these conditions tend to modify several 

 features of the prismatic structure, as already developed in what 

 precedes. Different parts also of the same large mass of basalt 

 will be found on comparing hand specimens to differ in grain, 

 hardness, &c, showing that the basalt when poured forth in a 

 liquid state was not perfectly homogeneous throughout. 



These differences involve variations of elasticity and rigidity in 

 different directions, and, we cannot doubt, variations in extensi- 

 bility under contractile strains, and in the rate of cooling under 

 given conditions. The principal effects produced by all these 

 are chiefly shown in variations of the mean diameter of the prisms 

 and in the length between the cross joints. Where the lengths 

 between these are usually found to be very great, as in most of 

 the Rhenish basalts, the prisms are found generally of small 

 diameter, often not exceeding six or seven inches ; and in such 

 cases the nearly flat cross joints, often many feet apart, are attri- 

 butable not so much to the differential contraction in cooling, 

 already described as generating cup-formed joints, but chiefly to 

 the propagation of mechanical strains through the whole mass, 



