758 AGE OF METAMORPHIC EOCKS. [Ch. XXXVH. 



CHAPTER XXXYII. 



ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS. 



Age of each set of metamorphic strata twofold — Test of age by fossils and mineral 

 character not available — Test by superposition ambiguous — Conversion of dense 

 masses of fossiliferous strata into metamorphic rocks — Limestone and shale of 

 Carrara — Metamorphic strata of older date than the Cambrian rocks — Others of 

 Lower Silurian origin — Others of the Jurassic and Eocene periods in the Alps 

 of Switzerland and Savoy — Why scarcely any of the visible crystalline strata 

 are very modern — Order of succession in metamorphic rocks — Uniformity of 

 mineral character — Why the metamorphic strata are less calcareous than the 

 fossiliferous. 



According to the theory adopted in the last chapter, the age of 

 each set of metamorphic strata is twofold — they have been deposited 

 at one period, they have become crystalline at another. We can rare- 

 ly 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. Superposition 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 crys- 

 talline texture in the cretaceous or in some tertiary period, the Eocene 

 for example. If in the latter, it should be called Eocene when regard- 

 ed as a metamorphic rock, although it be liassic when considered in 

 reference to the era of its deposition. According to this view, the 

 superposition of chalk does not prevent the subjacent metamorphic 

 rock from being Eocene. 



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, chlorite- 

 schist, quartz rock, statuary marble, and the rest. (See the two pre- 

 ceding chapters.) 



But when the metamorphic action has operated on a grander scale, 

 it tends entirely to destroy all monuments of the date of its develop- 

 ment. It may be easy to prove the identity of two different parts of 



