AGES OF METAMORPHIC ROCKS. 
597 
CHAPTER XXXV. 
ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS. 
Difficulty of ascertaining the Age of metamorphic Strata.—Metamorphic 
Strata of Eocene date in the Alps of Switzerland and Savoy.—Lime¬ 
stone and Shale of Carrara.—Metamorphic Strata of older date than the 
Silurian and Cambrian Rocks.—Order of Succession in metamorphic 
Rocks.—Uniformity of mineral Character.—Supposed Azoic Period.—Con» 
nection between the Absence of Organic Remains and the Scarcity of cal¬ 
careous Matter in metamorphic Rocks. 
According to the theory adopted in the last chapter, the 
metamorphic strata have been deposited at one period, and 
have become crystalline at another. We can rarely hope 
to define with exactness the date of both these periods, the 
fossils having been destroyed by plutonic action, and the 
mineral characters being the same, whatever the age. Su¬ 
perposition itself is an ambiguous test, especially when we 
desire to determine the period of crystallization. Suppose, 
for example, we are convinced that certain metamorphic 
strata in the Alps, which are covered by cretaceous beds, 
are altered lias; this lias may have assumed its crystalline 
texture in the cretaceous or in some tertiary period, the Eo¬ 
cene for example. 
When discussing the ages of the plutonic rocks, we have 
seen that examples occur of various primary, secondary, and 
tertiary deposits converted into metamorphic strata near 
their contact with granite. There can be no doubt in these 
cases that strata once composed of mud, sand, and gravel, or 
of clay, marl, and shelly limestone, have for the distance of 
several yards, and in some instances several hundred feet, 
been turned into gneiss, mica-schist, hornblende-schist, chlo¬ 
rite-schist, quartz rock, statuary marble, and the rest. (See 
the two preceding chapters.) It may be easy to prove the 
identity of two different parts of the same stratum; one, 
Avhere the rock has been in contact with a volcanic or plu¬ 
tonic mass, and has been changed into marble or hornblende- 
schist, and another not far distant, where the same bed re¬ 
mains unaltered and fossiliferous; but when hydrothermal 
action, as described in Chapter XXXIII., has operated gradu¬ 
ally on a more, extensive scale, it may have finally destroyed 
