Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/84

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Rum. Two distinct sets are perfectly visible both near Loch Scavig 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 distance 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 terminated 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.