90 GRANITE. 



occur, not exceeding a few yards in extent. In such 

 cases, it is probably the protuberant summit of a larger 

 mass beneath the surface; while in favourable situa- 

 tions, such a portion may gradually increase in extent, 

 from the degradation of the softer strata above it. 



It was already observed that granite might be in 

 contact with any rock of the primary series ; and, in 

 this case, its relative date to any member is proved by 

 the passage of veins through that stratum. If these 

 had been deposited by stratification upon it, the same 

 - contact of all the members might exist; but there 

 would be no intruding veins or disturbing masses. For 

 the same reason, even the secondary strata may re- 

 pose on granite without intervention of the primary ; 

 because it was not every where covered by these when 

 the former were deposited. In this case, the masses 

 or veins could not interfere with the strata ; and, should 

 that ever occur, such a mass of granite must be pos- 

 terior even to the secondary deposits. 



Tn^'/r-cw? The con tact of gneiss with granite, once supposed 

 " a indispensable, is so frequent, that instances need not be 

 quoted ; while there is often such a transition at the 

 place of meeting, both in general forms and in the 

 mineral structure, that the boundary is imperceptible. 

 In this connexion, the intrusion of veins is a frequent 

 feature. It is found in contact with micaceous schist 



I 'f C& 



'<d~A in several parts of Scotland, occasionally sending veins 

 into it ; while, as quartz rock occurs in the same si- 

 tuations, it is equally found in contact with that sub- 

 stance, and equally transmitting veins into it, as in 

 different parts of Aberdeenshire and Perthshire. Glen 

 Tilt and Shetland present examples of its intrusion 

 "into primary limestone. In Aberdeenshire, serpentine 

 ^ is often in contact with granite, as noticed in the hi- 

 story of that rock. Its contact with argillaceous schist 



