FORMATION OF BASALTIC DYKES. 



137 



of hornstone, and the gneiss has a red and burnt appearance, ap- 

 proaching in its nature to porphyry. It is probable that the action 

 of the basalt on the sides of the gneiss rock had softened it and ren- 

 dered it more liable to disintegrate than the other parts; for the sea 

 has here nnade an indentation inland, forming a deep narrow ravine 

 or bay, with a lofty wall of basalt running through it. The wall of 

 basalt completely divides the bay, and the sea enters on both sides of 

 the basalt. It has been before observed, that when basaltic dykes 

 extend into the sea, they form reefs of rocks, and small islands. 

 These basaltic walls, whether rising above the surface of the country, 

 or extending into the sea, serve to mark the destruction of the land ; 

 for we are certain, that these walls of mineral matter, were at one 

 period supported on each side by rocks or strata which they have 

 intersected, but which are now worn away. The Cleveland basalt 

 dyke, it has been stated, cuts through the transition limestone ; the 

 coal strata, and the upper secondary strata, comprising a part of the 

 oolite formation. On the northern coast of Ireland, Messrs. Buck- 

 land and Conybeare discovered a considerable basaltic dyke, pass- 

 ing through the chalk rocks. In the immediate contiguity of basalt, 

 the chalk on each side of the dyke was rendered highly indurated 

 and crystalline, this effect decreasing as the distance from the dyke 

 increased. 



The constant occurrence of dykes in basaltic districts, gives a high 

 degree of probability to the opinion, that overlying unconformable 

 trap rocks have been erupted through these dykes in a melted state 

 like lava, and have been poured over the surface of the ground. 

 Where extensive beds of basalt occur in low situations, there can be 

 little difficulty in admitting this mode of formation ; but the frequent 

 occurrence of beds of basalt, forming isolated caps on distant moun- 

 tains, was for a long time considered as opposing completely the hy- 

 pothesis of the igneous origin of basaltic rocks. A more attentive 

 examination of basaltic districts has however established the fact, 

 that these isolated caps of basalt are pans of extensive continuous 

 beds, which have in remote ages been excavated and intersected by 

 valleys, in the same manner as the beds of other rocks which fre- 

 quently form isolated caps on detached mountains. Isolated caps of 

 basalt are also, in some situations, formed like caps of modern lava, 

 which extend to no great distance from the summit of a volcano. 

 Professor Sedgwick of Cambridge, in an interesting description of 

 " The Phenomena connected with some Trap Dykes in Yorkshire 

 and Durham,^^ given in the Transactions of the Cambridge Philoso- 

 phical Society, states, that " In the quarries now excavating near Bo- 

 lum, the vertical dyke is unusually contracted in its dimensions ; but, 

 on reaching the surface, it undergoes a great lateral extension, espe- 

 cially on the south-west side, so that the works are conducted, in a 

 perpendicular face of columnar trap, more than two hundred feet 

 wide." The annexed cut (copied from that of Professor Sedg- 



IB 



