40 Dr Black on some appearances connected with the 



ances which we have noticed of rents and fissures will per- 

 haps not be so easy of explanation. However well many of 

 the faults and dislocations may be reconciled with the theory 

 of general subsidence, yet it appears to me not so easy to 

 associate any general part of the appearances we have noticed 

 with this view of geological disturbance, except the simple 

 fractures and rents which are observed without any appreci- 

 able width. A collapse, instead of an opening asunder, might 

 naturally be expected to be the result of a subsiding or cen- 

 tripetal force ; though, if a refrigerating process were at the 

 time or subsequently taking place, the surface of such subsid- 

 ing strata might, in suffering a corresponding contraction, be 

 rent or fissured, in frequency and extent varying with the 

 mechanical circumstances and chemical nature .of the beds in 

 question. 



The cleavage rents and interrupted lines of simple fracture 

 are generally considered to owe their determination to specific 

 arrangements within the rocky strata themselves — arising from 

 chemical or electrical affinities, or from what is more recently 

 termed molecular and polar arrangement. These lines, it is 

 well known, traverse the planes of stratification, difi:erently in 

 different rocks, but most in the same relative direction in the 

 same rocks. They at times cut the strata at right angles to 

 their planes ; at other times diagonally — forming, between 

 the lines of fracture or cleavage, either rectangular or rhom- 

 boidal sections. Professor Phillips has very industriously and 

 minutely examined these several lines of cleavage and other 

 fractures of disturbance, in his Geology of Yorkshire, so that 

 I shall not longer dwell on these preliminary matters, but pro- 

 ceed to claim a little attention to some other appearances 

 which are often to be observed in the same species and locali- 

 ties of rocks, as those which we have more particularly noticed 

 as being affected with lines of fracture, fissures, and disloca- 

 tions. 



'J'he appearances to which I allude are confined to the super- 

 ficial or uppermost strata of the sandstones in the Lancashire 

 and Derbyshire coal-formation, and especially consist in the 

 thinly-stratified or slaty sandstones of beds, varying from one 

 foot to six, to eight feet or more in dc^pth, of a laminated lulud 



