16 IONA. GEOLOGY* 



the granite with which it is so nearly in contact. The 

 sea prevents the actual and exact contact of the two 

 rocks from being very fully examined ; else it is probable 

 that a still more perfect series of this transition might 

 be observed. The contact of argillaceous schist with 

 granite is not rare in Scotland; but this is the only 

 instance I have witnessed in which the interference of 

 the latter is of such a nature as to produce the 

 appearance of a real transition from the one to the 

 other rock. 



Proceeding southwards along protuberances and cliffs 

 of a substance which holds an intermediate place between 

 clay slate and gneiss, a large mass of rock presents 

 itself in a most conspicuous manner from the almost 

 snowy whiteness of its surface, which is visible from a 

 considerable distance, even at sea. This white colour is 

 found on examination to proceed from the decomposition 

 of felspar, often so far advanced as almost to pass into 

 porcelain clay. The rock itself is an irregular body of 

 100 feet or more in thickness, and 600 in length. It 

 cannot be called a bed, but appears to be rather a 

 shapeless mass; having the same connexion with the 

 beds of rock in which it is involved, as we usually find 

 in serpentine, and not unfrequently in limestone, in similar 

 situations. It is a compact felspar, having the small 

 splintery fracture and imperfectly translucent appearance 

 at the edges, of a rock sometimes described by the name 

 of hornstone, which is a very frequent base of certain por- 

 phyries. It is extremely refractory to the hammer*. In 



* There are few geologists who have not been occasionally foiled in 

 their attempts to break such rocks with the hammers in common use. I 

 can, from long experience, recommend the following shape. It is either 

 spheroidal, or ellipsoidal, the largest diameter in the latter case not ex- 

 ceeding four inches. The weight need not exceed three pounds and a half. 

 It is evident that the centre ofgravity, and consequently the whole momen- 

 tum, will be so directed towards the point of contact, as in almost every 

 position to produce the maximum ctVect j a circumstance only accident- 



