86 ^n Essay on Rents. [Fe*. 



points to the reverse side of a perpendicular line, n o, to that to 

 which the angular direction in the second instance points ; and if 

 the stratum be bent till it be perpendicular, ilie side of the rent 

 where its surface of separation is situated will be horizontal. 



5> The Difference in the Position and Appearance of the Strata 

 letween where they are contiguous to, and at given Distances from, 

 these Rents. — The difference in the position of tlie strata is such, 

 that on the under side of all rents of this shape tb.ey are bent 

 upwards toward these rents ; and on their upper sides, downwards 

 toward them. This is the reason why miners never meet with two 

 parts of one stratum opposite each other on reverse sides of a rent 

 of this shape, but one part always a variaijle distance above the 

 other. The part of a stratum is very little bent downwards, to 

 whit it is bent upwards, towards this rent. Both parts of a stratum 

 are bent : the greatest distance opposite the middle part of a rent ; 

 and the least, near its highest and lowest extremities; and they are 

 also the least bent near the horizontal extremities, and the most at 

 the centre of this rent. These positions will be better understood 

 by havin;; reference to a diagram. The part. Of, of the stratum 

 A, fig. 1, Plate XXIX. which is opposite the centre of the rent, 

 ■ n w V, is bent upwards; the parts h, k, m, of the strata B, C, D, 

 are also bent upwards; but the furtiierthey are individually situated 

 from the middle stratum the less tliey are bent. Tiie same rules 

 hold good with the parts of the strata p, r, t. Again, tiie part 2; a 

 of the stratum. A, on the upper side of the rent, is bent downwards 

 a very little, but more than any of the strata, either above or below 

 it, as the ratio of the strata's bending decreases upwards, and 

 downwards from the stratum A, to the strata E and 1, which are 

 not bent near this rent. Again, the surfaces of separation of the 

 stratum A, are the greatest distance asunder at a b, fig. 1, Plate 

 XXX., or opposite the centre of the rent ; and the least, near its 

 horizontal extremities. A, « 1. The same rule applies to all the 

 strata opposite this rent, either above or below the stratum A. 



The appearance and hardness of that part of every stratum which 

 is close to this rent are different to the part of the same stratum 

 that is situated more remotely from it. These differences are owing 

 to corresponding differences in their proportions of elementary 

 matter; for on examining the strata we find one part of them has 

 sometimes one class * of elementary matter, and often one denomi- 

 nation, or more than one, of the class which the other parts want ; 

 and that the proportions of the different denominations of the 

 classes peculiar to both parts are different in one to what it is in 

 another part of a stratum. This arrangement of the elementary 

 matter of a stratum is found in different parts of all strata, but is 

 most obiervable near rents. These differences have been the most 



* The classes of (he earth's elementary matter are cicnerally divided into four — 

 •arths, metals, intiammables, and salts. Tlie diilcri'iit di'iiomiiiations of thes* 

 plas.-es may be sites, alumina, &c. ; iroM, copper, lead, &c. ; or bitumen, 

 carbon, &c. 



