on Basalt , &c. 
3*3 
extension of the aggregative force, and must be regulated by 
the texture, the form, and the position of the mass. Where the 
texture of the mass is homogeneous, and its contractions uni- 
form, its dimensions may be diminished, without its continuity 
being destroyed, provided its aggregation be so strong as to 
overcome the vis inertice of the mass, and its adhesion to other 
substances. But, when the resistance is sufficient to overcome 
the aggregation, the mass will be rent by fissures perpendicular 
to the direction in which the greatest resistance to its contraction 
takes place, or, in other words, by fissures perpendicular to its 
greatest surface ; for it is from the extremities of the greatest 
surface, that the largest quantity of matter must traverse the 
greatest space, in order that the contraction may be performed 
without breach of continuity ; therefore, if it be an extensive 
tabular mass, it will be divided into prisms, by fissures perpen- 
dicular to its surfaces The power of aggregation would de- 
termine these prisms to be hexagonal, as that form contains the 
greatest quantity of matter in the least surface, of any prisms 
that can be united without interposing prisms of other forms. 
But this would require the texture, the contraction, the thick- 
ness of the mass, and its adhesion to surrounding substances, to 
be every where precisely the same; and, as these conditions 
can never be fulfilled in an extensive formation, all the irregu- 
larities that are found must necessarily ensue. The same rule 
that determines the fissures of a tabular mass to be perpendicular 
to its surfaces, must determine the rents in a spheroid to be 
directed from its periphery to its centre. 
Though these considerations may be sufficient to explain the 
tendency to division into prisms, which is so generally extended, 
and which has produced many of those abortions that have been-. 
S s 2 
