332 GEOLOGICAL APPEARANCES 



gradations afford a presumption in favour of the igneous origin 

 of all granitic substances, that presumption must always dimi- 

 nish, as the affinity of the substance grows more remote; and 

 we have yet to wait for proofs, as strong as those wliich the 

 vakiable experiments of Sir James Hall have furnished for 

 trap, to be drawn from direct experiments, in the same way, 

 upon sienite and granite. 



127. Even admitting the sienite to have been introduced in 

 a state of fluidity from heat, it is still difficult to account for its 

 having completely filled up every interstice among the frag- 

 ments ; and it seems necessary to infer, that it flowed with a 

 strong impulse, while at the same time a pressure of superin- 

 cumbent matter, confining the adjacent bodies, opposed its in- 

 troduction. Indeed, if the sienite be inferred to have been 

 fluid from heat, compression must be admitted to have modi- 

 fied the action of that heat, since the carbonate of lime in the 

 sienite appears to have suffered no decomposition. If we 

 ascribe the fracture of the strata, the disorder in which the 

 fragments now lie among the sienite, and the wide separation 

 of some of them, to the force of the impelled fluid, we shall 

 adopt a simple, and connected, explanation of the phenomena. 

 In favour of this it may be further urged, that the fracture 

 and dislocation of the strata are confined to the neighbourhood 

 of the sienite, while those at a distance are comparatively un- 

 disturbed. 



128. Even close to the junctions, the large masses of strata 

 on the southern bank, similar in their component substances 



to 



the igneous origin of trap, cannot bear the idea of violating by any such hypo- 

 thesis the dignity of a rock, which has been so long venerated as the first founda- 

 tion of the earth. The specimens, required to exhibit the whole series, would 

 not be numerous, though the shades of difference, in every step, were slight. 

 Perhaps our island does not afford a better field for illustrating the subject than 

 Glen Tilt, and the mountains to the north-west of it. 



