68 yin Essay on Re?its. [Feb. 



advanced towards completion that if any separations took place in 

 it the parts into which it was separated could continue so. At that 

 time the attraction of cohesion in the middle parts of strata could 

 not be so powerful as to draw the matter towards their centres as 

 they diminished in bulk ; they would, therefore, be gradually 

 separated into masses of such dimensions that the attraction of 

 cohesion could draw the matter of each towards its centre from 

 every side as it continued to contract. 



The diminution of bulk, or the contraction of the strata, is very 

 evident from the rents or spaces between the numerous masses into 

 which they are divided. The contraction of strata horizontally can 

 be exactly ascertained. Thus if the length of the curved line, fly 

 be taken and laid in the direction f z, it will reach to c, and the 

 curve line, z a, similarly disposed will reach to d, so that there 

 will be a distance, d c, between them, which is the horizontal 

 contraction of the part, zf, of the stratum A. A distance between 

 similar parts of all other strata on the sides of this rent is also 

 observed ; the length of which is more or less, according as its 

 situation is more or less distant from the centre of the rent. And 

 although we have no direct proof that the strata have contracted 

 perpendicularly ; because in this direction its weight has no tendency 

 to counteract the force of cohesion, therefore no separation would 

 take place ; yet as the strata have contracted horizontally, we must 

 infer their contraction perpendicularly, as it is impossible to con- 

 ceive that they could contract in tiie former, and not in the latter 

 direction. But, as will be afterwards shown, because the contrac- 

 tion horizontally is greater in one part than in another, it is equally 

 probable that the contraction perpendicularly would be as unequal. 

 This is precisely the case : for a stratum lies in different levels on 

 opposite sides of all rents of this shape. Hence the strata on one 

 have contracted a greater distance perpendicularly than on the other 

 of their sides. 



The formation of the rents of this shape by the contraction, 

 unequally, of the matter contiguous to their sides is a consequence 

 resulting from the arrangement of the strata near them ; but the 

 ditFerences in the classes, and the proportions of each class of 

 elementary matter of the parts of the strata near the sides of rents, 

 when compared with those of these at given distances from them, 

 gave rise to the unequal contraction of the strata ; for the parts of 

 gtrata which differ from others in the proportions of their usual 

 classes of elementary matter must have contracted more or less than 

 them ; and those also which have a diiFerent class of matter, and 

 different proportions of the classes common to both, must have 

 contracted more or less also. Thus if the part of a stratum of coal, 

 with its usual proportions of elementary matter, was joined to a 

 part wanting a considerable quantity of bitumen, but had an extra 

 portion of earthy matter to make up this deficiency; these two parts 

 would not contract equally in passing from a fluid to a solid state, 

 and of course they would separate where they joined each other. 

 I 



