211 



longer presents a cube, but a figure materially reduced, 

 and its corners or angles rounded and irregular. 

 Thus if the process be frequently repeated, the cube 

 will be reduced to a spherical form. 



These are decidedly the specifick operations of wet 

 and dry, heat and cold, in the disintegration and de- 

 composition of all rocks, and other mineral substances, 

 susceptible of their effects. 



Of this fact we may, without going among the rocks, 

 see a striking illustration in that of, what is called, a 

 " slack burnt brick" exposed to their effects. In the 

 course of two seasons, a brick of this description thus 

 exposed, will be reduced from a parallelogram to an 

 oblate elipsis 



A similar effect is produced by the operation of heat 

 upon wood. Subject a piece of wood, in the form of a 

 cube, or that of a triangle, to heat, until it be ignited, 

 and the angles, from the same cause, are the first to be 

 on fire ; and if it be alternately ignited, and the carbon 

 removed to the depth at which it has penetrated, it will 

 at length be reduced nearly to that of a sphere. 



Taking these facts for granted, let us see how far 

 the several kinds of rocks, and, particularly, those of 

 the primitive formation, are operated on or effected by 

 those agents. 



1st. The old red, the ferruginous, and variegated 

 sand stones, are considered among those that are liable 

 to disintegration by frost, &c. 



If we examine these rocks in their natural or primi- 

 tive situation, they frequently present appearances 



