Essay introductory to Geology. 



13 



The third Lead of descriptive Geology of which we have to treat, will re- 

 late to the nature of the dislocations and disruptions which appear to have 

 affected the strata. To investigate the causes, indeed, of those dislocations, 

 may appear rather the business of theoretical Geology ; but simply to 

 describe them, is obviously the mere record of observation. 



§ 1. Now to render our statements clear, we must premise the re- 

 mark, that it is clearly evinced from the manner in which the shells, &c. 

 are imbedded in these strata, that they were originally deposited in a semi- 

 fluid state ; and as the materials of the matrix are generally found to have 

 penetrated into the minutest pores of those shells, its original state must 

 have been that of even more than semifluidity. Now it is self-evident, that 

 such deposits can originally have been precipitated only in flat horizontally 

 disposed masses ; but we actually observe the strata to be very seldom 

 strictly horizontal, and very frequently, especially in the rocks of the in- 

 ferior order, to have their planes inclined in very considerable angles, from 

 450 to 70° with the horizon, becoming sometimes quite vertical. The 

 case becomes still more striking, when we find there stripes of conglome- 

 rate rocks, which must have originally consisted of loose gravel, thus 

 vertically disposed. In the tertiary formation of Alum Bay, in the Isle of 

 Wight, we find beds still consisting of uncemented sand, with bands of 

 loose pebbles in this position : — 



Now the simplest laws of 

 gravity prove, that these ma- 

 terials could by no possibility 

 have been accumulated origi- 

 nally in their present position, 

 or in any other than horizon- 

 tal planes ; their actual situ- 

 ation must therefore necessarily be attributed to some species of dislocation. 

 § 2. Again, the necessary mechanical laws of their deposition must 

 evince, that any given series of deposits must originally have been ar- 

 ranged in continuous planes. But we actually often observe such a series, 

 after proceeding continuously for a considerable distance, interrupted by a 

 vertical cleft or fissure cutting across the rocks ; and we find the same 

 series repeated on the other side of that fissure, not in the prolongation of 

 the same planes, but with an alteration of level sometimes of only a few 

 inches, sometimes of one hundred fathom. It appears self-evident that 

 we must attribute such an arrangement to dislocation, attended with the 

 sinking of the whole rocky mass on one side of the fissure j or (which will 

 affect the same relative change of level) the elevation of the portion on the 

 other side. Such cases are called faults, a terra introduced by miners, 

 who having pursued a bed of coal to such a fissure, became at a fault how 

 to find its place on the other side. They are not, however, left without a 

 guide, for the extremities of the strata wliere broken off, are generally 



