142 POSITION AND NATURE, ETC. [Ch. VII. 



5. A granitic rock, unstratified ; forming, nevertheless, a 

 mass disposed in parallel beds, the thickness of which exceeds 

 a hundred feet. This rock consists of large-grained felspar, 

 green mica, and a very small proportion of quartz : it is in- 

 termixed with the above mentioned ores of iron, in small 

 groups, or nests ; and in the vicinity of these minerals, the 

 rock appears to be altered. 



6. This granitic rock is again succeeded by dolomite; 

 which is about twelve yards in thickness, and is not so regular 

 as the two preceding beds above described : its surface is not 

 even ; for the dolomite penetrates a little into the granitic 

 rock, and also into the granite on which it reposes. It still 

 contains spathose iron ore, but is particularly rich in various 

 kinds of iron glance. 



7. Lastly, we arrive at the granite which constitutes the 

 hills of St. Martin. It differs essentially from the rock 5, 

 being fine-grained with black mica : but, notwithstanding 

 this difference, we may rest assured that the granite rock 

 contained in the dolomite is a ramification from the granite.* 



The description of the junction of granite and limestone, 

 in this interesting locality, is somewhat blended with con- 

 jecture, and does not furnish such minute details as Glen 

 Tilt, owing, probably, to the sections being less favourable for 

 observation. We shall revert to this example when we 

 examine the nature of the evidence on which it has been 

 concluded that this granite must be of more recent origin 

 than limestone. 



Such is the condition of various primary strata at their 

 junctions with the granitic rocks ; and though they do not all 

 exhibit precisely the same appearances as those of Cornwall, 

 yet the points of resemblance are sufficiently numerous to 

 render it probable that the phenomena are of the same 

 nature, being only somewhat modified by local circumstances. 



* Bulletin de la Soc. Grfol. de France, tomeii. p. 71. 



