322 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV 



silica, whilst in a state of intense ignition, without entering 

 into combination therewith ? And if the temperature of the 

 quartzose mass be so elevated as to permit the formation of a 

 crystalline gneiss on its side most remote from the granite, 

 and also to reduce its supposed primeval state of sandstone 

 into a compact and crystalline quartz ; would not the union of 

 alkali therewith cause the whole to enter into a perfect state of 

 fusion, and could it be expected to find in this mass of glass, 

 when cooled, any trace of lines derived from stratification ? 

 It is unnecessary to add any further remarks to enforce the 

 value of this argument ; it speaks a plain and intelligible lan- 

 guage, which cannot be misunderstood. 



What shall we say, then ? Are the objections which have 

 been advanced frivolous, or are they sufficiently important to 

 induce geologists to enter on a careful scrutiny whether the 

 primary slates be really altered sedimentary deposits? If, 

 indeed, there be any weight in the foregoing arguments, the 

 primary schistose rocks are certainly not analogous to those 

 altered portions of secondary rocks adjacent to trap and other 

 igneous rocks ; nor is the transition of these crystalline slates 

 into fossiliferous strata indicative of a coeval formation : on 

 what grounds, then, it may be asked, can it be demonstrated 

 that the secondary rocks have been altered by granite, and 

 converted into gneiss, mica-slate, and other primary schists ? 

 The importance of this subject claims a more particular con- 

 sideration. 



"The slates surrounding Dartmoor have been changed," 

 says De la Beche, " by the intrusion of granite ; some being 

 simply micaceous, others more indurated and with the cha- 

 racters of mica-slate and gneiss, while others, again, appear 

 converted into a hard zoned rock strongly impregnated with 

 felspar. The alteration of the rocks in this case is of very 

 easy explanation. The greywacke, which is for the most part 

 the altered rock, is, when taken in the mass, only the conso- 

 lidated detritus of more ancient crystalline rocks, composed of 

 a few simple substances. If long-continued heat, insufficient 



