OF THE "VVESTEEN ISLES OF SCOTLAND. 367 



upon the evidence of the contact-metamorphism produced around 

 the intrusive mass of S. Airde Beinn (Sarsta Beinn), and the facts 

 then pointed out have been confirmed by Dr. Geikie and Dr. 

 Hatch *. 



In many cases the solfataric action and the invasion of the lavas 

 by great molten masses of rock can be shown to have produced 

 effects which are strikingly contrasted. The effect of the contact- 

 metamorphism is to induce a remarkable splintery fracture and 

 jointed structure in the rocks affected. But this effect is only seen 

 to extend to the distance of a few inches, or at most feet, from the 

 actual planes of junction. As the result of the contact-metamor- 

 phism the rocks acquire, in a very remarkable manner, a power of 

 resisting denudation ; and in consequence of this, we find sheaths 

 of altered rock standing up above the general surface, and enveloping 

 the intrusive masses f. 



The more widely spread changes which have affected the oldest 

 Tertiary andesites and diorites are of a totally different kind. The 

 action appears to have taken place in a sporadic and seemingly 

 capricious manner. Highly altered rocks may sometimes be found to 

 pass into comparatively unaltered rocks, within a few feet or yards, 

 and no direct relation can be detected between the greatly altered 

 masses and any particular intrusions of either acid or basic rock. 

 The effect of the chemical changes in the rock is usually to disin- 

 tegrate its constituents, and thereby render it less able to withstand 

 the action of denuding agents upon the mountain-sides. The che- 

 mically altered rocks, rendered soft and porous and coated with a 

 friable white crust, are often covered up and concealed by peat and 

 vegetation, while the intrusions among them, and their surrounding 

 sheaths produced by contact-metamorphism, retain the marks of 

 glacial action, and stand up prominently above the peat and 

 heather. 



In many places it can be clearly shown that the widely spread 

 chemical action has preceded the action of contact-metamorphism, 

 while in other instances the opposite may perhaps have been the 

 case. As might be expected, the results of these two kinds of action 

 are often curiousl}' complicated and involved. Microscopical and 

 chemical study enable us, however, in most cases to define and ex- 

 plain the exact nature of the results which follow from either kind 

 of action, and these I now i^ropose to consider. 



A, Kffecti< of Solfataric Action. 



To this cause must be assigned the alteration both of the pheno- 

 crysts (" Einspreninge '') and the ground-mass of the propylitic 

 rocks already described. 



The order in which tlie several changes take place is often 

 capable of exact definition, when a large series of sections made 



* Trans. Roy. Soc. Ediub. toI. sxxv. (1888) pp. 103, 104. 

 t Q. J. a. S. vol. XXX. (1874) pp. 2()5-206, figs. 4, 5, «& (5. 



