378 SKY. GEOLOGY. OVERLYING ROCKS. 



scribing the siliceous schists of Trotternish that they 

 often resemble basalt in appearance, the difference being 

 only marked by the slaty structure. It was also proved 

 that they were argillaceous schists altered by the influence 

 of the adjoining trap. It is easy to conceive that a further 

 continuation or a greater degree of this influence would 

 obliterate the structure also, and thus convert them into 

 simple stratified basalts. To apply this reasoning to 

 the western cliffs under review, it is only necessary to 

 suppose that the original schistose strata possessed the 

 variety of composition usual in similar cases, and that 

 they have been fused in their places ; the different beds 

 undergoing, in consequence, the various changes which 

 produced the differences in the trap strata now visible, 

 and the more infusible having been converted into 

 the siliceous schists that still remain to mark their 

 origin. There are many other cases of stratified trap 

 to which this explanation will apply ; and it is further 

 easy to see that it offers a solution of the difficulty before 

 stated respecting the connexion of that variety with 

 the mountainous one.* With this general account I 

 shall close the description of the great north-western 

 portion of Sky, and proceed to the north-eastern part, 

 which presents many interesting appearances. 



The trap already described, whether mountainous or 

 stratified, affords no examples, except in the very few 



* The porphyry which accompanies the coal field of Campbelltown 

 presents a structure very illustrative of this view. Through every 

 part of the mass, fragments of schistose rocks may be observed, 

 varying in dimensions and, in some cases, of considerable extent. 

 It is not difficult to trace the gradual transition by which they pass 

 into the shapeless mass that includes them, while they present 

 those other striking indications of the action of fire which I have 

 described at full length in a paper on the Hill of Kinuoul published 

 in the Geological Transactions. The mixture of schist and trap in 

 this hill is obviously of a similar nature, and equally illustrates the 

 present view. 



