FIC.13 



production of the Prismatic Structure of Basalt, 211 



to the walls of the fissure, and 

 separated by a stratum of irre- 

 gular fragments, which will be 

 in the middle of the dyke if 

 both walls of the fissure before 

 its filling were of equal con- 

 ductivity and at the same tem- 

 perature. The upper part of 

 the dyke will consist of a con- 

 fused assemblage of partly 

 prismatic, partly irregular frag- 

 ments, due to the irregularity of the cooling there, both from 

 the sides and the top*. If the mass have its top surface level 

 (but, as in all the cases refered to, covered with more or less irre- 

 gularities which do not affect the main result) and the mass abut 

 against a vertical or nearly vertical surface of rock or other solid 

 matter (as in fig. 14), there will be two ranges of prisms formed — 

 the one by cooling from the top, 

 straight and vertical, the other 

 by cooling from the abutting side, 

 straight and perpendicular to that 

 side ; and these two ranges, where 

 they would intersect each other, 

 will usually be separated by a 

 thinner or thicker stratum of irre- 

 gular fragments, inclined both to 

 the top and to the abutting sur- 

 faces, and dividing the entire 

 angle between these in a ratio 

 which depends on the rate of cool- 

 ing from the top and from the 



abutting side respectively, the prisms being longest in that 

 direction in which cooling has proceeded most rapidly — the ir- 

 regular fragments (in this as in all other instances in which they 

 are referred to) being produced by irregular cooling at the extremi- 

 ties of the prisms, where the wave of heat passes no longer in a 



* These conditions appear to militate seriously against the views pro- 

 posed by several authors as to the nature of those bright bands, often radia- 

 ting from one point, which are seen telescopically upon various parts of the 

 moon's surface— namely that these " Rillen " are dykes which have been 

 filled by injection from below, but the molten matter filling them has not 

 overflowed the lips of the fissures. Whatever might be the nature of the 

 molten matter filling these fissures, it is scarcely conceivable that the 

 broken-up top surface of the filling material should, after its consolidation, 

 reflect more light than the solid material of the sides of the fissures, if 

 there be any identity between the materials generally composing the moon's 

 surface and those forming that of our earth. 



P2 



