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 fossilliferous 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 M'ould 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 
