IN GLEN TILT. 349 



121. We may likewise recur to the argument, that veins of 

 these substances appear rarely in the strata of the adjacent 

 country, where no main rock of a crystallised aggregate is 

 near. 



122. It may farther be remarked, that the substance of the 

 main rocks of sienite is not quite uniform in its characters, for 

 in them the proportion of hornblende and quartz varies a 

 good deal, and is in some places small. A main rock at B was 

 mentioned, (parag. 58.) which, where in contact with strata of 

 compact dolomite with other ingredients, consists sometimes of 

 felspar only, for two or three inches from the line of junction. 

 A similar fact occurs at D. (Parag. 79.) While, however, we 

 admit, that the substances thus graduating into one another 

 were formed by crystallisation from the same fluid, the simi- 

 larity in the other circumstances of these veins forces us to 

 conclude, that some very trifling difference of cause was suffi- 

 cient to determine, whether hornblende and quartz should en- 

 ter into the composition of the vein, and in what proportions. 



123. It must at the same time be allowed, that there are cer- 

 tain veins of similar substances among these strata, of which it 

 may be doubted if they have been formed from the same fluid 

 mass with the main rocks of sienite ; such especially are the 

 large vein of white felspar on the east side of the bridge, and 

 the veins of red felspar with quartz, intersecting a rock of sie- 

 nite, and an imbedded mass of strata, at the foot of the north- 

 ern bank, below the bridge. 



124. The similarity of the stratified substances, lying in se- 

 parate masses among the sienite, to those which compose the 

 base of Ben y Gloe, and occupy a wide district to the south 

 and west, leaves no room for doubting that both are of the 

 same formation. But if this be admitted, and likewise the 

 common origin of the sienite in the main rocks, and in the 



VoL.VILP.IL Yy veins. 



