Ch. XIV.] OF THE STRATIFIED ROCKS. 329 



accordance with numerous facts which show that all the 

 crystalline rocks thus pass into the mechanical or secondary 

 deposits which have been formed of their detritus ; and not 

 only so, but even the unstratified and igneous granite exhibits 

 a similar transition into stratified conglomerate and sand- 

 stone, without the intervention of any crystalline slate: 

 facts which teach us that too much dependence must not be 

 placed on this evidence as a proof of identity of formation. 



The same reasoning is equally applicable to those cases in 

 which granite is supposed to have converted the secondary 

 strata into primary. The particulars of Hugi's examples, 

 referred to by Lyell, are not at hand : but the instances of 

 Glen Tilt in Scotland, and of the Eastern Pyrenees, detailed 

 in the sixth chapter, may serve for the present. 



In the first place, let us consider the metamorphosis which 

 the strata are supposed to have undergone at Glen Tilt, in 

 consequence of their coming in contact with granite. These 

 strata are, various kinds of schist, limestone, and quartz- rock. 

 The schists are clay-slate, hornblende-schist, talc-schist, and 

 mica- slate, which in some places approaches near to gneiss ; 

 but these are not disposed in any fixed or determinate order : 

 the hornblende -schist is often next the granite, but not always ; 

 the magnesian varieties are generally at some distance there- 

 from ; and the mica-slate and gneiss, which ought as the most 

 crystalline to be nearest to the agent of these changes, have 

 been produced where the influence of heat must have been 

 less energetic ; whilst portions of slate near the granite have 

 been, to use the prevailing phrase, altered in a less degree. 

 The limestone at a distance from the granite is very crystalline, 

 and either perfectly calcareous, or in some parts more or less 

 blended wifh and variegated by magnesian minerals : the 

 presence of magnesia is now generally attributed to the inter- 

 position of igneous rocks ; but here, as frequently happens 

 elsewhere, this is not always dependent on, or in proportion 

 to, the proximity of the granite. On the contrary, at the 

 junction of the limestone and granite, the former resembles 



