222 Mr. R. Mallet on the Origin and Mechanism of 



concretionary structure was due to the heterogeneity of the ma- 

 terial and the foreign nuclei introduced. The formation of such 

 curviform lumps is familiar to every ironmaster as occurring 

 occasionally in almost every variety of slag; but it is always 

 traceable to heterogeneity or to abrupt and unequal cooling. 

 Where is the evidence for any such concretionary development 

 in the vast masses of prismatic basalt, which are amongst the 

 most perfectly homogeneous of known rocks ? But assuming 

 that such a general structure did preexist, and that we had some 

 evidence in support of the purely arbitrary assumption that some 

 extraneous force existed sufficient to squeeze these to any extent 

 of mutual distortion, where is the evidence that such mutual 

 compression could even result in their being squeezed into con- 

 tinuous jointed prisms. The supposed concretionary masses can 

 only be assumed thrown confusedly together like cannon-shot of 

 various calibres in a pile, not even in regular layers or resting 

 on any symmetric base ; and as we ascend, the spheroids of each 

 succeeding layer must drop, so far as the variability of their sizes 

 will admit, to the lowest possible point between the supporting- 

 spheroids below. Each spheroid, therefore, as in the two only 

 possible arrangements of shot in a pile, which may have a trian- 

 gular or quadrangular base, must be in contact either with three 

 or with four supporting spheroids below it ; but in this case, 

 which is in reality the only possible one, mutual compression 

 (equal in all directions) will not produce prisms at all, but 

 merely an aggregate of rhombic dodecahedrons differently 

 posited in the two cases. If we are to have prisms as the result, 

 the spheroids must be assumed arranged in vertical or rectilineal 

 columns, each touching only one below it, in unstable equili- 

 brium and in defiance of all the laws which regulate the posi- 

 tions of free and mutually attractive molecules or gravitating 

 spheroids. 



If we admit these enormous improbabilities, then prisms either 

 tetrahedral or hexahedral might result from mutual compression ; 

 but, unless the spheroids were all precisely of equal diameter and 

 identical in compressibility and retained exactly their original 

 lineal arrangement during the whole progress of compression, 

 the prisms could not be even approximately of equal diameter 

 throughout. Nor could the cup-form of the cross joints ever be 

 produced at all, whether, as Mr. Scrope imagines, the convex 

 surfaces were always presented downwards, or the contrary ; for 

 it may be easily shown that the mutual distortion of such lines 

 of spheroids into prisms, whether square or hexagonal, must 

 result not in mutually fitting convexities and cups, but in ex- 

 tremely complicated opposite surfaces undulated both in radial 

 and circumferential directions. 



