294 



LYELL'S ELEMENTS OF GEOLOGY. 



Different Ages of Metamorphic Rocks. 



mainland ; 2dly, The abrupt truncation of dikes, such as those 

 at d (Fig. 293.), cut off on the surface of the granite; 3dly, The 

 fact that not only the secondary strata but the enormous masses 

 of trap which accompany and overlie them, terminate suddenly 

 on reaching the borders of the granite and schist, towards which 

 they often present a steep escarpment, and over which, for 

 some distance at least, they must originally have extended.* 



CHAPTER XXV. 



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 modern periods in the Alps of Switzer- 

 land and Savoy — Why the visible crystalline strata are none of them very 

 modern — Order of succession in metamorphic rocks — Uniformity of mineral cha- 

 racter — Why the metamorphic strata are less calcareous than the fossiliferous. 



According to the theory adopted in the 11th 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, 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 crystaUirie texture in the 

 cretaceous or in some 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. According to this view, 

 the superposition of chalk does not prevent the subjacent meta- 

 morphic rock from being Eocene. If, however, in the progress 



* In the WQrks of Drs.Hutton and MacCulloch, and in the Memoirs of Messrs. 

 Von Dechen and Oeynhausen, and that of Professor Sedgwick and Mr. Murchi- 

 son, (Geol. Trans. 2d series) and others, whose observations I have verified on 

 the spot, the reader will find a full description of the geology of Arran. 



