307

and limestone assume one figure; of basalt,another ; of slate, a third: they all breakin that direction in which the cohesion be-tween their several particles is least; theform of the block depends therefore on thenature of the substance, and the form ofthe fissure depends on the form of theblock.

According to the laws by which thecohesion of its particles is regulated, astratum in shrinking will divide into largeblocks or small. The pedestal of the statueerected to Peter the First was hewn outof a block which weighed 1500 tons. Howdifferent its dimensions to those of thefragments which we see re-united in a spe-cimen of ruin-marble from Florence, orjasper from Bohemia !

Fissures may also be produced or enlargedby the contraction of an adjoining mass,by the shock of an earthquake, or by failureof support, owing to the erosion of sub-terranean waters.

The fissures in Septaria can have beenoccasioned only by contraction, but con-

x 2