to Glen Cullader. 277 



the rock depend partly on tlie original texture and composition of 

 the mass, partly on the action of tlie atmosphere and of the sub- 

 terranean water and gases. When the felspar of the granite 

 contains little alkali or calcareous earth, and the mass is com- 

 pact, it is a very durable stone ; but when either the felspar 

 contains much alkahne matter, or the mica much protoxide of 

 iron, the action of the atmosphere and water containing oxygen 

 and carbonic acid, on the ferruginous and alkaline ingredients, 

 tends to produce the disintegration of the stone. As the nature 

 of the gaseous matters that rise through the fissures of the o^ra- 

 nite rocks in this quarter have not been examined, we cannot 

 say how far these may assist in breaking down the granite *. 



Glen Callader. — The granite continued to accompany us, with- 

 out any intermixture of other rock, until we reached an eminence 

 named Muckle Cairn-taggert, when large loose blocks, and even 

 fixed rocks of hornblende-rock and slate made their appearance. 

 These were traversed by veins of granite, often of considerable 

 size. These rocks are probably ranged alongside the granite, 

 and may be a continuation of the slate-rock observed at the 

 bridge over the Dee, at the foot of the LionVHead. We now 

 descended into the wild Glen Callader, and walked by the river 

 which flows thi-ough it to the mouth of the glen, where Callader 

 water flows into the Clunie water. The bed of the river exhi- 

 bits numerous displays of the phenomena that occur where gra- 

 nite is associated with slaty primitive rocks, and, for its extent, 

 no glen in Scotland is more remarkable, not even the famous 

 Glen Tilt. We examined with great care a succession of alter- 

 nations of slate rocks, as common hornblende-slate, micaceous 

 hornblende-slate, mica-slate, and gneiss with apparent beds of 



• It is worthy of notice, that the proportion of carbonic acid in the at- 

 mosphere, according to the observations of Saussure, the son, are not always 

 the same. Over a wet soil, the atmosphere contains less carbonic acid than 

 over a dry one ; more carbonic acid exists in the air during tlie night than 

 during the day ; the superior strata of the atmosphere contains more carbonic 

 acid than the inferior ; and, lastly, a violent wind generally augments the 

 carbonic acid in the lower strata of the atmosphere, during the day, by the 

 intermixture of the lower and upper aerial strata, and sometimes by the wind 

 blowing from a dry quarter. 



