354 Geological appearances 



buoyed them up, and raised them to their actual position *-. 

 Nor can this be explained even by supposing, that these mas- 

 ses of hornblende-slate were attached to others of gneiss, and 

 limestone ; for, while the heaviest specimen of sienite has the 

 specific gravity of 2.67, the lightest of two specimens of gneiss 

 was found to have that of 2.68, and the lightest of three speci- 

 mens of limestone that of 2.70. A fluid impelled from below, 

 though of a less specific gravity, might raise them ; and that 

 they should have remained suspended till the sienite crystallis- 

 ed round them, may be accounted for by the supposition, that 

 the sienite was in that state of viscid fluidity, which earthy 

 bodies, when melted, are known to assume at a temperature 

 not far above their point of crystallisation ; joined to the consi- 

 deration, that the difference of specific gravity could n.ot be 

 great. 



130. Any theory ascribing the origin of the sienite to an in- 

 troduced fluid, of whatsoever kind, would lead us to account 

 for the gradations, which in some places appear upon the lines 

 of junction, by supposing that the stratified masses were acted 

 upon by the fluid, so as to admit of their being penetrated by 

 it, or at least by some of its ingredients. After having ascrib- 

 ed the fluidity of the sienite to heat, it is obvious to refer the 

 mutual action of the substances to the same agent, and to sup- 

 pose that it softened the strata, so as to enable certain ingredi- 

 ents of the fluid to penetrate them to a small depth. Some 

 instances of gradation, where carbonate of lime is one of the 



substances, 



* The large projecting rock below the bridge furnishes a similar argument. 

 The specific gravity of one specimen of hornblende-slate, from the top of this 

 rock, was found to be 2.95, that of another 2.97: the specific- gravity of its sie- 

 nite was not tried, but the mean specific gravity of two specimens of sienite 

 from D is 2.66. 



