276 An Essay on Rents. [April, 



masses of one sort vary in denomination as do the strata; but those 

 of the other are generally of one denomination throughout a rent, 

 and their arrangement is very diflferent from that of the first sort. 

 These masses I divide, for i-easons which will afterwards appear, into 

 the first-formed and second-formed tabular masses. 



Of the First-Formed Tabular Masses. 



The first-formed tabular masses are known by the names of vein- 

 stones, ribs, walls, &c. In the parts of these rents where the 

 first-formed tabular masses exist very regularly arranged, the fol- 

 lowing order is observed. If A A I, fig. 1, Plate XXXII., be a 

 stratum of coal, in the rent H I a tabular mass of coal G is so situated 

 that its under side m d lies between two lines d, m, one parallel to 

 and opposite the line which joins the surfaces of separation b e to 

 the under side ef, and the other parallel to and opposite the line /, 

 and its upper side a i is similarly disposed towards tlie lines /', h, as 

 is its under side towards the lines e, /. If the stratum B B 1 b^ 

 slate-clay, it has a tabular mass of slate-clay w in the rent, similarly 

 situated with respect to it, as the mass G is to the stratum A A 1 ; 

 the stratum C, and every other stratum in the rent above the stratum 

 A, has a tabular mass arranged in the same manner as the masses 

 G O ; and the strata D, E, F, and all the strata below them, 

 have tabular masses A, q, r, &c. which are similarly disposed. But 

 all the masses differ as much from one another in external charac- 

 ters, as the strata from which they proceed difl'er from one another. 

 The arrangement of the first-formed tabulat masses is seldom so 

 regular as it is here represented : for sometimes there are many 

 small tabular masses between the surfaces of separation of the strata, 

 ss fe, fig. 2, represents. These masses are sometimes at great 

 distances from one another, in the horizontal direction of the rent, 

 and verv small ; but as we proceed horizontally towards the centre 

 of the masses, as in the direction a b, they increase in dimensions, 

 and at the centre, as at b, there is one large mass, which perhaps 

 fills the whole height, and a considerable distance horizontally. 

 Beyond this mass they decrease in dimensions, and as the distance 

 from it increases, the distance from one another increases also ; till 

 at last they become what miners aptly call " grimings." Beyond 

 this place, as beyond c, we find no more of these masses for a con- 

 siderable distance cd; they then begin again, increase and decrease 

 in magnitude as before. 



The arrangement of the first-formed masses, as just described, is 

 very perfect in all small rents of this shape j but in the larger it is 

 seldom perfect. 



The first-formed tabular masses, as I said before, resemble tlie 

 matter contiguous to the rents in which they are situated ; so that 

 in the coal formation they consist of alternations of coal, slate- 

 clay, wliitie sand-stone, &c. ; in the red, and white sand-stone, of 

 red and white sand-stone; in lime-stone, of lime-stone; and in 

 gninitr, gncis, sienite, &c. of these rocks respectively. 



