488 On Alineral and Metallic Veins. 



McCulloch's Western Islands, it will be observed, that the hori- 

 zontal trap veins represent the handle and triple prongs of a 

 fork ; and that if all that part of the section to the left, from 

 where the handle is joined to the prongs, had been disintegrated 

 and worn away in the lapse of time, leaving the part to the 

 right representing the prongs ; or if the same part of the section 

 had been so covered up with other mineral matter, as to defy 

 examination, the part exposed would have constituted a very 

 puzzling case of horizontal trap veins ; but we can here trace 

 the prongs to the handle, and the handle to a huge vertical 

 dyke of trap that has its undoubted origin from below : there 

 is also a smaller vertical shaft rising in the handle, and three 

 ramifications which the handle appears to have given out. 



Figure 2, plate xiii. is a section of Loch Eyshort, also in the 

 Isle of Sky. Here the trap, for a great extent, like the palisa- 

 does on the Hudson river, the rocks near New Haven, and those 

 at the Passaic in New Jersey, spreads in extensive masses over 

 the surface of the ground ; and according to the old Wernerian 

 opinions, was deposited from aqueous solutions. In this section,^ 

 however, we have a satisfactory view of the origin of this trap, 

 which every locality does not give ; for we see the roots of the 

 trap, and have no room left for doubt that its origin is from be- 

 low. Fig. 3, plate xiii. is another instructive section, represent- 

 ing the intrusion of a vein. Here we perceive how the expan- 

 sive force from below, has raised the strata on each side ; and 

 how the fissure or vein which divides the two masses, contains 

 various fragments of them, embedded near its edges. These 

 jets of trap which have been thrown up from below, have ob- 

 tained the name of dykes in England, and are, in some instances, 

 remarkable for their great extent, and for the number of beds 

 which they intersect. The celebrated Cleveland Dyke, of which 

 we have spoken at page 343, (see April No.) extends about a hun- 

 dred miles in the northern counties of England. At Preston 

 quarry on the Tees, it comes up through the new red sandstone; 

 at a quarry at Langburgh it cuts through the Lias, and at Bo- 

 lam quarry, it not only comes up through the coal measures, 

 but overflows the surface, as is represented by fig. 4, plate xiii. 



We have shown, at page 311, the perfect agreement between 

 the mineral constituents of modern lava, trap, and greenstone, 

 one of the primary rocks ; with such a strong indication of their 



