1815.] An Essay on Rents. 8J 



accurately examined in common coal. Sometimes we find that 

 part of a stratum of coal which is contiguous to this rent contains 

 only carbon and small portions of iron, and earthy matter ; while 

 the parts more remote contain carbon, bitumen, and the other in- 

 gredients peculiar to common coal. Again : near this rent the coal 

 often contains ten per cent, of earthy matter ; but this is not the 

 case with it further from the rent : and coal sometimes has its 

 portion of iron increased to five or six per cent., but wants its 

 bitumen ; thc-n its appearance and consistence approaches very near 

 to those of graphite, or black lead. * The ditferences in the 

 appearance of one part of every stratum, when compared with that 

 of another, is very various ; and the nature of these difterences in 

 clifTerent strata is as various ; but in this particular they all agree 

 Hvith one anotlier, that the parts near the rents which differ from 

 the characteristic parts of the strata have the least degree of hard- 

 ness, and would he the least able to resist any force that may have 

 acted against tlie attraction of cohesion. All the strata are not 

 wholly ditTerent near this rent to what they are at given distance* 

 from it. We pass through a rent in many places near its centre, 

 sides, or higher or lower extremities, without perceiving an altera- 

 tion in the strata on either side of it ; but after much examination 

 of them, I have satisfied myself that there is the following general 

 law respecting them : the parts which are different to the rest on 

 the under side of the rent are generally in the greatest abundance 

 opposite its lower half, but these parts are opposite the higher half 

 •on the upper side of this rent. In fig. 1, Plate XXIX., the lines 

 bf, fe, and e v, will reuresent the extent of the differing parts 

 opposite the lower half, and the lines a\, 12, and 2 n, the extent 

 of these parts opposite the upper half of this rent. 



6. The fi,nnwg Cause of this Rent deduced from the foregoing 

 Facts.— When we consider that the positions of the strata above 

 and below the small bended- tabular rents, have not been altered by 

 the force which altered the positions of those on their sides, and 

 that it has altered the positions of these for small distances oidy, we 

 must conclude, that had it acted either from above or below the 

 rents, or from either of their sides, it could not have formed them, 

 by producing such alterations in the positions of the strata adjoining 

 them. VVc must, therefore, define the forming cause of this rent, 

 one that has acted only in the altered parts of the strata. This 

 cause could be no other than the matter's contraction, unequally, in 

 changing its state from fluidity to solidity. 



During the change of the matter from fluidify to solidity, there 

 would be a period of time when its consolidating process was so far 



• Many tucli diin-renre? in oilier Mr.ita lia»e been ilcsfribcd by minrralogisti. 

 The pail, wliiclKliUor vi-rv inurli fnim llic rcbt ilii-y have .iii.|>..,i(l to \u- paiU 

 rhaiigrd hy somi- clirmi.ai ii;,'.iil ; but I consi.lei llinii as ur/.?»i«/ din-reiics and 

 lh< ca.i .; of all iPrits; lH-cau!.e (li<- strata, in the parlj v.tn .lill.-rriit fioni tin- rr.t, 

 woul-l dimiiiMi in bulk, I si ur mvr* in ilie foriuei i>art. than in the latter, a> i« 

 alt rwufiib di-xribed. 



