378 RELATIVE AGE OF [Ch. XXVI. 



secondary periods. It is very possible, for example, that con- 

 siderable tracts of hypogene strata in the Alps may be altered 

 oolite, altered lias, or altered secondary rocks inferior to the 

 lias ; but we can scarcely ever hope to substantiate the fact, 

 because, whenever the change of texture is complete, no cha- 

 racters remain to afford us any insight into the probable age of 

 the mass. Where granite happens to have intruded itself in 

 such a manner as partially to overlie a mass of lias or other 

 strata, as in the case before alluded to (diagram No. 90, p. 371), 

 we may prove that fossilliferom strata have become gneiss, 

 mica-schist, clay-slate, or granular marble ; but if the action of 

 the heat upon the strata had been more intense, the same infer- 

 ences could not have been drawn. It might then have been 

 supposed that no Alpine hypogene strata were newer than the 

 carboniferous period. 



The metamorphic strata of Scotland are certainly in great 

 part older than the carboniferous, which are found incumbent 

 upon them in an unaltered state ; but it appears that secondary 

 deposits as new, or newer than the lias, have come in contact, 

 in the Western Islands, with granite, and have there assumed 

 the hypogene texture. 



A considerable source of difficulty and misapprehension, in 

 regard to the antiquity of the metamorphic rocks, may arise 

 from the circumstance of their having been deposited at one 

 period, and having assumed their crystalline texture at another. 

 Thus, for example, if an Eocene granite should invade the 

 lias and superinduce a hypogene structure, to what period shall 

 we refer the altered strata ? Shall we say that they are meta- 

 morphic rocks of the Eocene or Liassic eras ? They assumed 

 their stratified form when the animals and plants of the lias 

 flourished; they became metamorphic during the Eocene period. 

 It would be preferable in such instances, we think, to consider 

 them as hypogene strata of the Eocene period, or of that in 

 which they were altered ; yet it would rarely be possible to 

 establish their true age. We should know the granite, to 

 which the change of texture was due, to be newer than the lias 



