1.40 



PROTRUDED BASALT. VOLCANIC DYKES. 



intermixture of green-earth, toadstone, and limestone, near the junc- 

 tion of toadstone with the limestone beds, certainly favors Mr. White- 

 hurst's original theory of protrusion ; but this protrusion took place 

 before the formation of metallic veins, and might be the cause of 

 those fissures in which the veins were formed. It is not improbable 

 that some of the more regular beds of toadstone may have flowed 

 as lava. Professor Sedgwick justly observes, " that our reluctance 

 to admit the theory of protrusion arises from the difficulty of con- 

 ceiving any powers in nature adequate to produce such an effect. 

 But all the phenomena of Geology show, that the great disturbing 

 forces by which the crust of the globe has been modified, acted in 

 former times with incomparably more energy than they do at present. 

 Volcanic forces are now employed in lifting a column of melted lava 

 to the lip of a crater. The same kind of forces, acting with more 

 -energy and through a wider region, may in the early history of the 

 globe have been employed in lifting islands and even continents from 

 the bottom of the ocean. During an operation like this, the elastic 

 forces, acting from below, may often have driven masses of fluid lava 

 among the superincumbent strata ; and, in every case, the lava would 

 naturally be propelled through those portions which were most easily 

 penetrated— the lateral must, at every point, have been equal to the 

 vertical pressure. The expansive forces may not at any point have 

 been able to drive a column of lava through all the solid unbroken 

 beds, but the lateral forces may have driven a portion of the fluid 

 between the partings of two horizontal beds; and when a penetra- 

 tion of this kind was once effected, the lava like a wedge, would act, 

 to mechanical advantage, and rush, in an horizontal stream, to a dis- 

 tance proportioned to the elastic forces which were in action." 



The formation of basaltic dykes is sufficiently explained by what 

 takes place in the vicinity of volcanoes. Before the confined vapour 

 that afterwards issues through the crater, finds a vent there, the sur- 

 face of the ground in the vicinity of the volcano is frequently up- 

 heaved, and fissures of great extent are made, into which melted 

 lava is sometimes forced, which on cooling forms a wall or dyke, in 

 every respect similar to a basaltic dyke. During an eruption of Ve- 

 suvius that took place in 1794, a rent of this kind was formed near 

 the bottom of the mountain, 2375 feet in length and 237 feet in 

 breadth, which became filled with compact lava. Rents or fissures 

 of some miles in length have been opened on the sides of Etna. 

 There is abundant evidence to prove, that most basaltic rocks were 

 erupted under the pressure of the ocean, and it is probably owing to 

 circumstances attending their refrigeration, that they have frequently 

 a columnar structure. 



The occurrence of thick beds of basalt, divided into regular pen- 

 tagonal or hexagonal columns, and disposed in ranges of vast extent 

 and height, could not fail to arrest the attention of the most careless 

 observer, and give rise to speculations respecting their origin and for- 



