Volcanos. 127 



u The circumstances which determine the time occupied by 

 lavas in cooling, will depend on the figure of the mass, external 

 circumstances, and the structure and composition of the lava. In- 

 stances are quoted of currents retaining a great heat for a con- 

 siderable time. That of Jorullo, in Mexico, is by no means cool 

 yet, though produced in 1759. 



" The next chapter treats of the divisionary structure assumed 

 by lavas on their consolidation. This process must be accom- 

 panied, at all times, by a diminution of volume or contraction. 

 Were it to commence at the center of a mass no separation 

 of parts need take place ; but if at the surface, different centers of 

 contraction must establish themselves, and fissures of retreat be 

 formed between them. The figures these circumscribe, tend to 

 approximate to the hexagon. But since there is no opposition to 

 the contractile force, in a direction perpendicular to the surface, 

 which subsides freely as the mass below contracts, no fissure, or 

 very few will be formed parallel to the surface ; and by the in- 

 ward propagation of the retreat, the hexagons will be lengthened 

 into hexagonal prisms. The slower the process of solidification, 

 and the finer the grain of the lava, the more regular will be the 

 prisms, ceteris paribus ; hence the interior, or lowest parts of a 

 current alone, in general, require this structure, which does not 

 become visible till denudation has exposed these parts. This is 

 rarely the case with recent lavas ; and hence arises, according 

 to our author, the common error of supposing the columnar di- 

 vision confined to the older basalts. This structure is very fre- 

 quent in dikes, both in the older and recent volcanic formations ; 

 the columns being always perpendicular to the sides of the dike. 

 When lava rests on a convex surface, the columns diverge ; 

 when on a concave, converge upwards ; being always perpendi- 

 cular to the surface on which the process first acts. The author 

 remarks, that those of the peaks of basalt, which are so nume- 

 rous in basaltic districts, will be found to consist of a group of 

 convergent columns ; this disposition affording the maximum of 

 resistance to the action of rain and frost, in separating the co- 

 lumns, and breaking up the bed, of which the remainder has pro- 

 bably been destroyed in this manner. More than one kind of 

 divisionary structure may occur in the same rock ; smaller prisms 

 are sometimes formed within the large. The globiform struc- 

 ture is next accounted for, and its occasional subdivision into 

 radiating prisms, or concentric leaves. The angulo-globular 

 structure accompanies a tendency to the formation of globular 

 concretions. The tabular, lamellar, and slaty, or schistose divi- 

 sionary structures, are supposed to be confined to lavas in which 

 the crystalline particles are disposed more or less conformably, 

 owing to which their mobility is considerable in the direction of 



