76 Dr. Mac Culloch's Sketch of the 



Rum. Two distinct sets are perfectly visible both near Loch Scavfg 

 and at Strathaird, and the examples are unquestionable, since those 

 of one period hold their course through the other in every direction, 

 with the same pertinacity and distinctness as the first do through 

 the fundamental rocks. We have no means of knowing what dis- 

 tance of time has intervened between these veins. The angle of 

 their courses with the horizon is various, but in a very considerable 

 proportion it is vertical or nearly so. 



They are of frequent occurrence in the mica slate of the district 

 of Sleat, and, as far as I have seen them, they are here basaltic 

 They also abound In the sandstone at Loch Eishort, where they are 

 of considerable size. From this bed they are readily traced through 

 the superincumbent ones as far as the most distant surface of the 

 limestone, and here I have always lost them. In several attempts 

 for that purpose, I never could discover their continuation through 

 the syenite, and am therefore inclined to think that they are prior 

 to It. To ascertain this fact, I caused a portion of the marble bed 

 which was traversed by two of these veins to be cleared away to 

 its contact with the syenite, and found that the whole mass termi- 

 nated together against it, leaving in my mind little doubt that the 

 syenite was posterior to the veins. This fact has a double bearing : 

 it might be argued that the stratified rocks reposed on the syenite, 

 since the nature of the contact between stratified and unstratified 

 rocks Is always such as to admit of a double interpretation, unless 

 where veins of the one are seen decidedly ramifying from its mass 

 into the other. But if, as in this instance, the trap vein, which we 

 know to hold In all cases an unchecked progress through opposing 

 rocks, Is cut off by the unstratified rock, the syenite, it is plain that 

 the stratified rock which that vein traverses has also been broken 

 by It, and that the syenite in this particular case is posterior to the 

 limestone, and of course to all that body of rock which precedes it. 



