BASALT, AND TRAP. 47 



the mineral characters of these dikes present insensible 

 gradations, from a state of compact lava, through infinite 

 varieties of greenstone, serpentine, and porphyry to granite, 

 we refer them all to a common igneous origin. 



The sources from which the matter of these ejected 

 rocks ascends are deeply seated beneath the granite ; but it 

 is not yet decided whether the immediate cause of an erup- 

 tion be the access of water to local accumulations of the 

 metalloid bases of the earths and alkalies ; or whether lava 

 be derived directly from that general mass of incandescent 

 elements, which may probably exist at a depth of about one 

 hundred miles beneath the surface of om* planet.* 



Our section shows how closely the results of volcanic 

 forces now in action are connected, both wdth the pheno- 

 mena of basaltic formations, and also with the more ancient 

 eruptions of greenstone, porphyry, syenite, and granite. 

 The intrusion both of dikes and irregular beds of unstrati- 

 lied crystalline matter, into rocks of every age and every 

 formation, all proceeding upwards from an unknown depth, 

 and often accumulated into vast masses overlying the sur- 

 face of stratified rocks, are phenomena coextensive with 

 the globe. 



Throughout all these operations, however turbulent and 

 apparantly irregular, we see ultimate proofs of method and 

 design, evinced by the uniformity of the laws of matter and 

 motion, which have ever regulated the chemical and me- 

 chanical forces by which such grand efl^ects have been pro- 

 duced. If we view their aggregate results, in causing the 

 elevation of land from beneath the sea, we shall find that 

 volcanic forces assume a place of the highest importance, 

 among the second causes which have influenced the past, 

 as well as the present condition of the globe ; each indi- 

 vidual movement has contributed its share towards the final 

 object, of conducting the molten materials of an uninhabita- 



• See Cordier on the internal temperature of the earth. 



