50 DIKES AND VEINS. 



with heat as an essential agent : for this agency will explain some facts in the history of 

 veins and dikes, which, on the common theory of fusion from dry heat, were inexplicable. 

 Thus, when we find veins not thicker than writing paper (and those of granite, epidote 

 &c., are sometimes as thin, and some of trap are less than half an inch), it is difficult to 

 see how they could have been filled by injection of melted rock, especially if the walls 

 were not very hot ; but by means of water the materials could be introduced wherever 

 that substance would penetrate. Again, in the silurian rocks of Vermont, on the shore 

 of Lake Champlain, we find numerous dikes, both of greenstone and trachytic porphyry. 

 These dikes are in some cases partially filled with a conglomerate, or breccia, composed of 

 limestones, sandstones, gneiss, quartz, and granite, of the rocks in fact that occur in the 

 region, as we shall describe in another place in detail. Now the limestone fragments 

 have lost none of their carbonic acid. But this would have been driven off, as in a lime 

 kiln, if the dike had ever been heated to redness, or to 1000 of Fahrenheit ; for carbon- 

 ates are decomposed below that temperature. This is all consistent if the partial 

 plasticity of the dike was aqueo-igneous ; but inexplicable if dry heat alone were con- 

 cerned. Moreover, such dikes must have been filled mechanically from above, and this 

 might have been done by the currents of an ocean, sweeping into the crevices on its 

 bottom the rounded pebbles accumulated there. . 



8. The facts of metamorphism teach us tJiat most rocks have undergone several entire chan- 

 ges since their original production. Take the unstratified rocks. These have all been cooled, 

 and most of them crystallized from a state of fusion, either entirely igneous, or aqueo- 

 igneous. Here is one change : but from the vertical movement of the isothermal line in 

 the earth's crust, and the erosions at the surface, probably all the original igneous rock 

 has been re-melted and re-cooled much of it perhaps several times. If any rock has 

 escaped this second fusion, we know not where it is to be found. 



Take the stratified rocks. These being derived by abrasion from the unstratified, have 

 of course passed through the just mentioned changes. But abrasion has brought them 

 under another, a mechanical change, and water has collected the fragments together at the 

 bottoms of lakes and oceans. Subsequently, by consolidation and some chemical agents, 

 transfused by water through the mass, it has become converted into cletrital fossiliferous 

 rock. Buried afterwards beneath vast accumulations of other rocks, the heat has 

 increased and hot water has permeated the strata, reducing them to a state more or less 

 plastic, causing a crystalline to take the place of a mechanical structure, obliterating ' the 

 fossils and substituting cleavage or foliation for lamination. In some cases the heat 

 might be so great that all traces of stratification are blotted out, and granitic or trappean 

 rocks are the result. It may be after all this, that erosion has again converted these rocks 

 into detritus, and the process of deposition and of metamorphism begins again : nor can 

 we tell how many times these changes may have been repeated. When they have passed 

 through one cycle of change they are as fresh as ever to commence another. 



9. The final conclusion is, that the entire crust of the globe has undergone metamorphism, 

 and is not now in the condition in which it was created. We are sure that every part of it 

 has been in a molten state ; and we have every reason to suppose that every part of it has 

 gone through other changes ; nor is there any evidence that a portion of the first consoli- 

 dated crust remains. 



