306 GEOLOGICAL APPEARANCES 



primary strata. Where observed by the side of the Tilt, it was 

 between 20° and 30° ; in the bed of the Banavie, it was 

 32°. 



4. On the western bank of the Tilt, between two and three 

 miles above its junction with the Garry, we observed some sie- 

 nite in a scar, by the side of a walk through the Duke's plan- 

 tations ; and in another, not far from it, some gneiss. The 

 soil that covered the neighbouring ground, prevented us from 

 ascertaining the geological relations of the sienite. The gneiss 

 is singular for having its component substances so disposed, 

 that the structure of the stone bears no marks of stratification ; 

 but its structure differs from that of granite, in being less per- 

 fectly granular, and less highly crystallised. 



5. The strata in the bed of the Banavie, a little below a 

 bridge, that crosses it about a mile above Blair, are cut by a 

 vein, or dyke, of felspar-porphyry. The felspar base is chiefly 

 of a red colour, but partly also grey. The vein is vertical, 

 and about ten feet broad *. 



6. There is a good road carried up Glen Tilt, as far as Fo- 

 rest Lodge, one of the Duke of Atholl's hunting-boxes, nearly 

 seven miles from Blair. This road, after crossing two or three 

 small streams, in the beds of which there is mica-slate, de- 

 scends, about the end of the second mile, to the side of the ri- 

 ver. There is here a bridge, called Gilbert's Bridge, and about 

 fifty yards below it, we found in the channel gneiss, interstra- 

 tified with granular limestone. From the bridge the road fol- 

 lows the bottom of the valley, and being often carried along 

 the bank of the river, affords good opportunities of examining 



the 



* Dr Hotton mentions, (in a manuscript intended for his third volume, and 

 aever published) that he found several dykes, both of grey and red porphyry, in 

 the course of this stream. 



