Mr. Bbtce on the Geological Structure of Roseneath. 



117 



A little way north of this point, and at another bend in the vein, the 

 outer or salient angle is intersected by a dike of greenstone and basalt, 

 in such a manner that a portion of 4 



the felspathic rock is isolated be- 

 tween the whin dike and the mica 

 slate, and the continuation of the 

 vein lies on the same side of the 

 dike as before the intersection. 

 The annexed sketch, also a ground 

 plan, shows the mode of this inter- 

 section, which is the most singular 

 I have ever met with. 



The dike is very distinctly trace- 

 able for several hundred yards to- 

 wards the north-east, the surface 

 occupied by it rising into conical 

 hummocks. It is then lost in 

 marshy ground for a short distance, 

 but is again continued towards the 

 mountains. In the other direction, 

 after a lengthened and most careful 

 search, its course was satisfactorily 

 made out as far as Portincaple 

 Ferry, where it is well seen ; and I 

 have met with a dike of the same 

 width and bearing, near the top of the mountain on the east side of 

 Loch Eck, traversing mica slate and altering it considerably ; which, I 

 have no doubt, is a prolongation of this dike. The width is about 25 

 yards, and it bears a point S. of W. It is in many places inclined at 

 the same angle as the slate, among the beds of which it seems to 

 have insinuated itself in a serpentine course. The mica slate in some 

 places is slightly changed by the contact, being rendered harder and 

 more massive: the lamination is partially destroyed, and the rock is 

 banded, parallel to the sides of the dike. In one place pieces of the slate 

 are seen enclosed in the dike, and slightly altered. Portions of the wedge 

 shaped mass of slate, d, between the two veins, are entangled in the basaltic 

 dike, and altered in the same manner. 



At the edges the dike consists of blue slaty basalt, but the greater 

 part of the mass is a coarse grained greenstone, which at several points 

 exhibits in great perfection that peculiar structural arrangement in 

 concretionary spheroids, which is the most frequent and characteristic 

 form assumed by the trap rocks, and of which the columnar is but the 

 result, when under favourable conditions, they parted more slowly with 

 their heat of fluidity. The best marked of these is in a cliflf about 60 

 feet high, overhanging the marshy ground above mentioned, where the 

 dike has a considerable underlie, the slate being in contact on both sides. 



a a Vein offelsjmikic rock, 

 h h Altered schist. 

 c c Dike of greenstone and basalt, 

 d d Mica slate. 



