CHAP. xiv. IGNEOUS ROCKS. 201 



the footpath leading to the bridge has been cut. The 

 wandering geologist approaches, and just as he is about 

 to step on to the bridge, to look down upon the raging 

 torrent below, his attention is arrested by the interesting 

 phenomenon of the primary or igneous rock lying in con- 

 tact with the slate or secondary rock. The molten 

 matter seems to have forced its way up through the 

 clay slate, bending it as easily as the potter does his 

 clay ; and the heat has fused it into mica slate. 



" Not only are the strata in contact with the granite 

 altered to gneiss and mica slate, but about the centre of 

 the mass a piece of black mica is seen, with a vein of 

 different-coloured granite leading to and beside it, 

 suggesting the idea that this black mica had at one 

 period been a piece of ordinary schist, which had got 

 entangled in the molten matter as it rose, and thus 

 assumed the appearance which it now exhibits. I broke 

 a piece right out of it, and will find an opportunity of 

 sending it to you. I also took a piece of red granite for 

 you, and a piece of gneiss. The gneiss is most interest- 

 ing in situ : it is bent into a beautiful curve. Such and 

 such is the fact, if the metamorphic theory be the correct 

 one ; indeed, the metamorphic men could hardly find a 

 better argument than in this case. 



" After the river passes this bridge its channel becomes 

 rugged in the extreme. Then you come to Dirlot Castle 

 a picturesque ruin on a granitic rock, about thirty feet 

 over the river's channel a very romantic spot !"* 



* Some of the scenes through which the river Thurso passes, especi- 

 ally in the upper pails of the parish of Halkirk, are full of romantic 



