566 



ROCKS ALTERED BY 



[Ch. XXXIII 



ing, are of modern origin, in which crystals, not only of mica, but of 

 quartz, are common, together with felspar and hornblende. It is easy 

 to conceive how such volcanic masses may, at a certain depth from the 

 surface, pass downwards into granite. 



I have already hinted at the close analogy in the forms of certain 

 granitic and trappean veins ; and it will be found that strata penetrated 

 by plutonic rocks have suffered changes very similar to those exhibited 

 near the contact of volcanic dikes. Thus, in Glen Tilt, in Scotland, al- 

 ternating strata of limestone and argillaceous schist come in contact with 

 a mass of granite. The contact does not take place as might have been 

 looked for, if the granite had been formed there before the strata were 

 deposited, in which case the section would have appeared as in fig. 686 ; 

 but the union is as represented in fig. 687, the undulating outline of the 

 Fisr. 686. Fig. 687. 



Junction of granite and argillaceous schist in Glen 

 Tilt. (MacCulloch.)* 



granite intersecting different strata, and occasionally intruding itself in 

 tortuous veins into the beds of clay-slate and limestone, from which it 

 differs so remarkably in composition. The limestone is sometimes 

 changed in character by the proximity of the granitic mass or its veins, 

 and acquires a more compact texture, like that of hornstone or chert, 

 with a splintery fracture, and effervescing feebly with acids. 



The annexed diagram (fig. 688) represents another junction, in the 

 same district, where the granite sends forth so many veins as to reticu- 

 late the limestone and schist, the veins diminishing towards their termi- 

 nation to the thickness of a leaf of paper or a thread. In some places 

 fragments of granite appear entangled, as it were, in the limestone, and 

 are not visibly connected with any larger mass; while sometimes, on 

 the other hand, a lump of the limestone is found in the midst of the 

 granite. The ordinary colour of the limestone of Glen Tilt is lead blue, 

 and its texture large-grained and highly crystalline ; but where it ap- 

 proximates to the granite, particularly where it is penetrated by the 

 smaller veins, the crystalline texture disappears, and it assumes an ap- 

 pearance exactly resembling that of hornstone. The associated argilla- 

 ceous schist often passes into hornblende slate, where it approaches very 

 near to the granite. f 



* Geol. Trans., 1st series, vol. iii. pi. 21. 

 f MacCulloch, Geol. Trans., vol. iii. p. 259. 



