Ch. XI.] THE PRIMARY ROCKS. 223 



stance is the appearance in the shale of numerous crystals of 

 analcime and garnet, which are distinctly confined to those 

 portions of the rock affected by the basalt. This discovery is 

 most interesting, because the garnets do not exist in the 

 unaltered shale and limestone ; and have, therefore, evidently 

 been produced by heat, without effacing the marks of stratifi- 

 cation : thus supporting the hypothesis that mica-slate, which 

 abounds in garnets, is an altered sedimentary rock. Again, 

 at Stirling Castle, the sandstone next the greenstone is in- 

 durated, and at the point of junction has assumed a texture 

 approaching to hornstone. In the north of Ireland, the chalk 

 next basaltic dikes is converted into a granular marble, and 

 all traces of organic remains are effaced. And when these 

 dikes pass through the coal measures, the shale is converted 

 into flinty slate ; and, in one instance, a bed of coal is reduced 

 to cinder, for the space of nine feet on each side. 



Let us now return to the stratified rocks next granite ; and 

 we shall find them, in such position, also presenting an 

 appearance different from those parts of the strata more 

 remote. Thus, schistose gneiss becomes massive, and not to 

 be distinguished from granite, with the exception of lines of 

 stratification, which, however, are sometimes very obscure. 

 Mica-slate becomes gneiss under similar circumstances ; and 

 clay-slate passes into hornblende-schist. 



This is not only the case with the primary stratified rocks, 

 that is, those that are anterior to the greywacke and carbo- 

 niferous groups, but the secondary beds of slate in the Alps, 

 referrible to the age of the lias and oolite formations, are 

 described by M. Hugi as having been converted into gneiss 

 and mica-slate. 



The Plutonic theory supposes that, in these cases, the heat 

 communicated by the granitic mass, reduced the contiguous 

 strata to semi-fusion ; and that, on cooling slowly, the rock 

 assumed a crystalline texture. The experiments of Gregory 

 Watt prove that a rock need not be perfectly melted, in order 

 that a re-arrangement of its component particles should take 



