478 VARIOUS FORMS OF [Ch. XXIX. 



from 5 to 1 feet in width, which will afford a scale of measurement for 

 the whole. 



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, 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 Skye, is no less than 

 100 feet in width. 



Fig. 629. 



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



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

 stone, 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 the 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, 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 ver- 

 tical in its main direction, slightly flexuous, but about one foot 

 thick. On each side are walls of compact 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 wider after the lava on 

 each side had consolidated, and the additional melted matter poured 

 into the middle space may have cooled more rapidly than that at 

 the sides. 



In the ancient part of Vesuvius, called Somma, a thin band of half- 

 vitreous lava is found at the edge of some dikes. At the junction 

 of greenstone dikes with limestone, a saklband, or selvage, of serpen- 

 tine is occasionally observed. On the left shore of the fiord of Christi- 

 ania, in Norway, I examined, in company with Professor Keilhau, 

 a remarkable dike of syenitic greenstone, which is traced through 

 Silurian strata, until at length, in the promontory of Naesodden, it 



