Cii. XXXVin AGE OF METAMORPHIC ROCKS. 611 



CHAPTER XXXVII. 



ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS. 



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

 eral 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 — ^YhJ scarcely any of the visible cr3'S- 

 talline strata are very modern — Order of succession in metamorphic rocks — > 

 Uniformity of mineral character — Why the metarxiorphic strata are less cal- 

 careous than the fossiliferous. 



According- to tlae 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 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. Superposition itself is an ambiguous test, espe- 

 cially when we desire to determine the period of crystallization. SujDpose, 

 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 som^ 

 tertiary period, the Eocene for example. If in the latter, it should be 

 called Eocene when regarded as a metamorphic rock, although it be 

 liassic when considered in reference to the era of its deposition. Accord- 

 ing 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, w^e 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 preceding 

 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 difierent parts of 

 the same stratum ; one, where the rock has been in contact with a vol- 

 canic or plutonic mass, and has been changed into marble or hornblende- 



