181 5.J An Essay en Refits. 079 



have adapted themselves to the inequalities of the latter, those must 

 have been in a fluid state when these were solid. Hence the first 

 obtained their situations after the second sort of masses. With the 

 help of these observations, it is easy to account for the forming^ 

 cause of the second-formed tabular masses. As the bended tabular 

 rent continued to increase in height and width, fluid matter entered 

 it, and the first-formed tabular masses were produced in the manner 

 already described ; but after a certain length of time, no more 

 matter was separated from that contiguous to the rent; yet by the 

 contraction of this matter, the rent continued to widen, and the 

 first-formed masses contracted also, so that hollow spaces were 

 formed in it. These spaces could not lie without some interrup- 

 tions as to their existence opposite the whole of the superficial area 

 of a rent; because some of the first-formed masses must lie against 

 the sides of the rent, to retain their situations, and to support 

 others which did not rest' against the sides. The spaces so formed 

 were gradually and slowly filled up in the manner of a clayey sedi- 

 ment, and thus produced the second-formed tabular masses. 



2, Of the Matter in Rents containing Earthy and Metallic- 

 Tubular Masses, 



The arrangement of the earthy-tabular masses in rents which 

 contain metallic-tabular masses is the same in every respect as in 

 rents filled only with earthy-tabular masses, with the addition of 

 being associated with the metallic masses which are found some- 

 times on the sides and middle of the rent, and which are subject to 

 the same variation of thickness, and want of continuity, as the 

 earthy-tabular masses. There is one circumstance in tlie arrange- 

 ment of metallic matter which is peculiar to formations consisting 

 of alternating strata of difierent denominations, 'J'he metallic 

 masses are not found in rents oj)posite strata of every denomination, 

 hut only opposite one or two. Thus, in the mining distiict round 

 Alston, in Cumberland, the strata consi-st of white sand-stone, 

 lime-stone, coal, slate-clay, &c. ; but tlic lead ore is only found in 

 lime-stone and sand-stone, and never opposite the rest; except 

 when the fiist-formed masses of lime-stone or sand-stone are 

 situated in a rent, a little lower than the strata from wiiich they 

 proceed ; then tliey sometimes have masses of lead ore adiiering to 

 them ; and when the lead ore is found in small masses mixed 

 throughout similar masses of lime-stone, sand-stone, and clav-slate, 

 it is sometimes lying opposite the clay-slate strata ; but here 

 it is evident, from the lime-stone and sand-stone masses which 

 accompany it, that it has fallen from opposite the lime-stone and 

 sand-stone strata, 'J'he same peculiarity t;ikes place in the J)crby- 

 shire mines. 'J'hire the lead ore is found only opposite the lime- 

 stone, and not opposite the amygdaloid, or the slate-clay strata, 

 except when accon)i)anicd with lime-stone masses. 



As- there are no metallic strata opposite most of the l)ended- 



