400 SKY. GEOLOGY. TRAP VEINS. 



steatite being there disposed in the same manner where 

 the veins intersect the calcareous sandstone. This con- 

 nexion is attended by a change in the substance of 

 the trap by which it approaches to serpentine; the 

 whole moreover appearing to be connected with the pre- 

 sence of limestone, since the change in question takes 

 place only where those two rocks interfere. As this 

 fact is important in the history of serpentine, a rock of 

 which the natural affinities are but little understood, 

 it may be useful to add that an instance of the same 

 nature, but much more decided, occurs in Perthshire, 

 where a trap vein traverses a mass of limestone in a 

 similar manner. In this case the transition from the 

 trap to the serpentine is very perfect ; a line of the 

 latter substance occupying the outer part of the vein 

 while the limestone in contact with it is also filled with 

 steatite. 



In describing the limestone of Broadford I formerly 

 remarked that it contained beds of trap, often so equably 

 interstratiiied as to be generally undistinguishable from 

 regular alternations. An excellent example of their real 

 nature, and of their identity with the analogous appearances 

 in the north-eastern coast of Trotternish, is afforded by 

 a circumstance occurring among similar beds at Borrereg. 

 In one of these the bed, after a very extensive parallel 

 course among the strata of limestone, undergoes a sudden 

 flexure into an oblique position ; which shortly becoming 

 vertical, it is then continued beyond reach of investigation 

 under the usual form of a common trap v^in ; intersecting 

 at right angles in one place the strata to which it was 

 parallel in another. 



The last trap vein which appears worthy of specific 

 notice is found near Loch Oransa traversing the gneiss. 

 It is remarkable for the mixture which it presents within 

 itself of all the ordinary varieties of trap : being a fine 

 basalt at the edge and passing by degrees into green- 

 stone, porphyry, and amygdaloid. 



