IN GLEN TILT. 351 



tion of the sienite was going on. But an objection to any 

 idea of a crystalline deposition from an aqueous fluid, arises 

 from the compactness of the rocks consisting of stratified mas- 

 ses intersected by the sienite. This circumstance has been ur- 

 ged by Mr Playfair, as an objection to the aqueous origin of 

 granite rocks * ; and the argument may be appHed with greater 

 force to the veins of sienite in Glen Tilt. Had an aqueous 

 solvent deposited this substance among fragments, in some 

 places so confusedly heaped, in others so closely fitted, toge- 

 ther, we might liave expected to have found occasionally what 

 so frequently occurs in a calc-tuf, a porous structure, and ca- 

 vities, perhaps lined with crystals. On the contrary, in all 

 these veins, however complex and minute, scarcely a single in- 

 terstice appears unfilled. 



126. It is to heat then that we may with more probability 

 ascribe the fluidity of the sienite. This hypothesis is farther 

 supported by the affinity of sienite to trap, or whinstone ; and, 

 in proportion to that affinity, we may extend to sienite the in- 

 ference of an igneous origin, which, with respect to trap, seems 

 established by so many convincing arguments. The crystallis- 

 ed aggregates of Glen Tilt afford excellent examples of 

 gradations between these substances, and the first steps of 

 the series from trap may be traced in the sienitic green- 

 stones of the northern side of the valley ; some of which 

 very nearly resemble the most highly crystallised specimens 

 of common greenstone from Salisbury Craigs. The sie- 

 nite in the bed of the river may be considered again as a step 

 in a continuation of the series to granite f. But, though these 



Yy 2 gradations 



» See Hutlonian Theory, art. 81. 



t This series of gradations, in composition as well as structure, between 

 trap and granite, does not appear to have been duly noticed ; and I would parti- 

 cularly recommend it to the attention of those geologists, who, while they admit 



the 



