300 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



it does not follow that these correspond to the planes of 

 deposition ; so the liassic slates and limestones of the Botz- 

 berg may successively present themselves at the surface, 

 exhibiting layers which decline towards the granite, and yet 

 the entire mass of each of these rocks may be wedge-shaped, 

 basin-shaped, or of any other form, ranging even in the op- 

 posite direction. The term stratum, and all its properties of 

 bearings and dip, both longitudinal and lateral, have been so 

 variously and indefinitely used, that we cannot form any 

 correct idea of the relative position of rocks, unless all the 

 minute circumstances of the case have been carefully de- 

 tailed. If, in addition to this difficulty, it be remembered 

 that the earthy materials of the liassic slates have very pro- 

 bably been derived from the debris of the crystalline rocks, 

 immediately or through the medium of a previous migration ; 

 and, since this debris uniformly contains scales of mica, 

 more or less abundant, it is not surprising that a mechanical 

 transition should here occur ; but, at the same time, we cannot 

 admit this as infallible evidence that the mica-slate and 

 gneiss are identical with the strata of lias, only differing 

 therefrom in consequence of having assumed a new aspect, 

 superinduced by their contact with granite in a state of 

 incandescence. 



It is time, however, to discuss the capability of caloric to 

 effect such metamorphoses, a topic which can be more satis- 

 factorily approached, since the arguments advanced in its 

 support depend more on facts than opinions. " The geo- 

 logist," says Lyell, " has been conducted, step by step, to 

 this theory, by direct experiments on the fusion of rocks in 

 the laboratory, and by observation of the changes in the 

 composition and texture of stratified masses, as they approach 

 or come in contact with igneous veins or dikes. In studying 

 the latter class of phenomena, we have the advantage of 

 examining the condition of the rock at some distance from 

 the dike, where it has escaped the influence of heat, and its 

 state where it has been neai' to or in contact with the fused 



