production of the Prismatic Structure of Basalt. 225 



radial planes passing through its axis — contrary to the result of 

 observation in all natural basalt. Were it worth while to oc- 

 cupy space in further refutation of this notion, it would be easy 

 to point out many other incongruous conditions either involved 

 in the conception or in its consequences. One other notion is 

 to be found more or less obscurely suggested by various authors, 

 and to which even some countenance has been given by Delesse ; 

 it is that which supposes basaltic prisms to have resulted from 

 crystallogenic forces, and to be themselves of the nature of 

 gigantic crystals. 



The mere magnitude of the prisms is not alone sufficient to 

 overthrow this notion; for undoubted crystals of quartz, of beryl, 

 carbonate and sulphate of lime, galena, common salt, &c. are 

 known to exist of magnitudes vying with the diameters at least 

 of basaltic prisms. But these all possess the character of true 

 crystals, viz. that their internal structure is throughout that an- 

 nounced by their exterior contour; but in no basaltic prism, 

 however distinctly crystallized may be the integrant particles of 

 dissimilar minerals (pyroxene, felspar, &c.) of which the prism 

 consists, is there the slightest relation traceable between the di- 

 rections of the axes of these integrant crystals and the axis or 

 contour of the prism itself. We might as well call a hexagonal 

 column of granite or porphyry cut out by the hand of a lapidary 

 a crystal, as to apply the term to a basaltic prism. 



But though crystalline forces have had no part in the produc- 

 tion of basaltic prisms, the laws of their production by symme- 

 tric fracturing, due to cooling and contraction, do present some 

 interesting analogies with the results of crystallization under 

 certain circumstances in bodies solidifying from fusion. 



It has been pointed out by the writer, in his paper " On 

 the Physical Conditions involved in the Construction of Artil- 

 lery " (in the ' Transactions' of the Royal Irish Academy, vol. xxii. 

 June 1855), that in all such cases the principal axes of the crys- 

 tals constituting the cooled mass arrange themselves in direc- 

 tions orthogonal to the external contours of the mass — that is, 

 in the directions in which the heat-wave has passed from the 

 cooling body; that is to say, the principal axes of the integrant 

 crystals are in their arrangement very similar to those of the 

 axes of the prisms in prismatic basalt. And the analogy extends 

 even to this — that the principal axes of the crystals are curved 

 where the crystallizing mass has curved surfaces of contour, or 

 where the volume in relation to the cooling surface changes more 

 or less abruptly; in other words, where the isothermal surfaces 

 of cooling are divergent, as, for example, in the ogival portion 

 of elongated projectiles for rifled cannon of chilled cast iron, 

 the principal axes of the crystals are found curved, as in fig. 24, 



Phil. Mag. S. 4. Vol. 50*. No. 330. Sept. 1875. Q 



