§ 27. TRAP DIKES IN THE ISLE OF SKY. 857 



Sky the intrusions of basalt are on a large scale, and pre- 

 sent many important and instructive examples of the dis- 

 turbance and altered character of the sedimentary rocks, 

 that have B*een exposed to their influence. From the 

 numerous sketches that illustrate Dr. Macculloch's work on 

 the Western Isles,* I have selected the one before us 

 (Lign. 194), as exhibiting vertical, oblique, and horizontal 

 veins or dikes ; a large mass of trap is seen abutting end- 



Lign. 194. — Trap-dike on the coast of Trotternish, m the Isle 

 of Sky. 



a, Vertical Trap. 



b, c, d, trap-veins sent off from the mass a. 

 e, strata of sandstone. 



wise at a, against the sandstone strata e; from which a 

 thick stream flows horizontally, sending off branches both 

 upwards (b) and dowmvards (d), and finally dividing into 

 three small veins (c, c, c). 



In the cliffs at Straithaird, in the Isle of Sky, the sand- 

 stone strata are traversed by numerous vertical dikes and 

 veins of trap ; and the latter in many places have decom- 

 posed, and left perpendicular fissures, as is shown in the 

 annexed sketch {Licjn. 195); the reverse of the pheno- 

 mena observable in the Val del Bove (ante, p. 829). 



Porphyritic dikes and veins also occur abundantly in the 

 same island, in some instances protruding through, and in 

 others spreading over clay-slate, red- sandstone, and shelly 

 limestone. 



* This work should he referred to in order to obtain an adequate 

 idea of the extent and complexity of the trap-dikes and veins in the 

 Isle of Sky, and others of the Hebrides. 



