Ch. XXIX.] 



TRAP DIKES AND VEINS. 



611 



to 7 feet in width, which will afford a scale of measurement for the 

 whole. 



Ground plan of greenstone dike traversing sandstone. Arran. 



In the Hebrides and other countries, the same masses of trap 

 which occupy the surface of the country far and wide, concealing 

 the subjacent stratified rocks, are seen also in the sea cliffs, pro- 

 longed downwards in veins or dikes, which probably unite with 

 other masses of igneous rock at a greater depth. The largest of the 

 dikes represented in the annexed diagram, and which are seen in 

 part of the coast of Skye, is no less than 100 feet in width. 



Fig. 6S1. 



Trap dividing and covering sandstone near Suishnish in Skye. (MacCulloch.) 



Every variety of trap-rock is sometimes found in dikes, as basalt, 

 greenstone, felspar-porphyry, and trachyte. The amygdaloidal traps 

 also occur, though more rarely, and even tuff and breccia, for the 

 materials of these last may be washed down into open fissures at the 

 bottom of the sea, or during eruptions on the land may be showered 

 into them from the air. 



Some dikes of trap may be followed for leagues uninterruptedly 

 in nearly a straight direction, as in the north of England, showing 

 that the fissures which they fill must have been of extraordinary 

 length. 



In many cases trap at the edges or sides of a dike is less crystal- 

 line or more earthy than in the centre, in consequence of the melted 

 matter having cooled more rapidly by coming in contact with the 

 cold sides of the fissure ; whereas, in the centre, where the matter of 

 the dike is kept longer in a fluid or soft state, crystals are slowly 

 formed. But I observed the converse of the above phenomena in 

 Teneriffe, in the neighborhood of Santa Cruz, where a dike is seen 

 cutting through horizontal beds of scoriae in the sea-cliff near the 

 Barranco de Bufadero. It is vertical in its main direction, slightly 

 flexuous, and about one foot thick. On each side are walls of com- 

 pact basalt, but in the centre the rock is highly vesicular for a width 

 of about 4 inches. In this instance, the fissure may have become 



