PART I. CHAPTER VIIL 



105 



Various forms of Trap Dikes and Veins. 



Ground plan of greenstone dike traversing sandstone. Arran. 



which occupy the surface of the country far and wide, conceal- 

 ing the subjacent stratified rocks, are seen also in the sea-clifFsj 



Fig. 93. 



Trap dividing and covering sandstone near Suishnish in Sky. (MacCuUoch.) 



prolonged 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 Sky, is no less than 100 

 feet in width. 



Every variety of trap rock is sometimes found in these dikes, 

 as basalt, greenstone, felspar, porphyry, and more rarely tra- 

 chyte. The amygdaloidal traps also occur, 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 uninterrupt- 

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

 showing that the fissures which they fill must have been of ex- 

 traordinary length. 



Dikes more crystalline in the centre. — In many cases trap at 

 the edges or sides of a dike is less crystalline 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, the matter of the dike being 

 kept long in a fluid or sofl state, the crystals are slowly formed. 

 In the ancient part of Vesuvius a thin band of half- vitreous lava 

 is found at the edge of some dikes. At the junction of green- 

 stone dikes with limestone, a sahlband, or selvage, of serpentine 

 is occasionally observed. 



On the left shore of the fiord of Christiania, in Norway, a re- 

 markable dike of syenitic greenstone is traced through transition 

 strata, until at length, in the promontory of Nsesodden, it enters 



