300" Sir J Ames Hall on the ComoHdation of the Strata. {Oct: 



the certainty of many important facts has in consequence been 

 ascertained beyond dispute, which were formerly unknown, or 

 at least involved in such obscurity, that no person could have 

 ventured to assert them, without being charged with extrava- 

 gance. But though, no doubt, many branches of this science 

 still remain to be investigated, such inquiries may now be said 

 to have acquired a considerable degree of consistency and in- 

 terest, from the substantial basis upon which they have been 

 found to rest. 



Thus, in the present day, it is universally admitted, that a 

 -great part, I believe, in point of bulk, by far the greatest part, 

 of the solid rock which constitutes the external mass of our 

 ■globe, is stratified : that these strata, or at least a considerable 

 portion of them, have at one period consisted of a loose assem- 

 blage of sand and gravel, broken from rocks of still higher anti- 

 quity : that these fragments are infinitely various in quality, in 

 bulk, and in form ; some retaining their original sharpness, 

 others rounded and polished by agitation in the water : that 

 these beds alternate with others of limestone, composed, in a 

 great measure, of the shells of sea-fish, which shells are also 

 ♦occasionally scattered through the other strata. So that on the 

 whole, it seems to be sscertained to the satisfaction of all par- 

 ties in geology, that the strata, — those, at least, of later forma- 

 tion, have once constituted collections of incoherent parts. 

 And it is further admitted, that these beds have undergone va- 

 rious remarkable changes, some chemical, some mechanical. 



The chemical changes consist in the consohdation of these 

 •loose assemblages into their present state of rock, passing, in 

 frfiat transition, through boundless varieties, in point of flexibility 

 and toughness, and occasional brittleness. The mechanical re- 

 yolutions are no less remarkable, principally in the change of the 

 strata to their present contorted shape, and elevated position, 

 often many thousand feet above the surfiice of the sea ; though 

 there is fiiU reason to believe that they all once lay in a hori- 

 zontal position at its bottom. • 



I have said that the greatest part of the crust of our habitable 

 globe seems unquestionably to be stratified, and produced from 

 detritus or fragmented materials. The other portion, though 

 j)robably the least in bulk, is, generally, the most conspicuous, 

 «owing to its durability, elevation, and picturesque beauty. This 

 iind of rock is contrasted with the former class, particnlarly in 

 it^ negative qualities ; in being, according to some geologists, 

 .ultogiether devoid of stratification in the general mass, and en- 

 tirely /ree from component fragments ; the whole being made 

 *ip of crystalline forms, moulded upon each other, in obedience 

 to certain chemical laws. 



This crystalline rock, as the Society are well aware, abounds 

 >nthe neighbcm^hood gf ildinburgh, iii Arthur's Seat, Salisl^ury 



