CONTENTS. 



CHAPTER XXIV. 



ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS. 



Difficulty in ascertaining the precise age of a plutonic rock Test of age by 

 relative position Test by intrusion and alteration Test by mineral compo- 

 sition Test by included fragments Recent and Pliocene plutonic rocks, 

 why invisible (p. 283.) Tertiary plutonic rocks in the Andes Granite 

 altering cretaceous rocks (p. 286.) Granite altering Lias in the Alps and in 

 Sky Granite of Dartmoor altering Carboniferous strata Granite of the Old 

 Red sandstone period Syenite altering Silurian strata in Norway (p. 289.) 

 Blending of the same with gneiss Most ancient plutonic rocks Granite 

 protruded in a solid form (p. 291.) On the probable age of the granite of 

 Arran in Scotland. 



CHAPTER XXV. 



ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS. 



Age of each set of metamorphic strata twofold (p. 294.) 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 Switzerland and Savoy (p. 296.) Why the visible crystalline 

 strata are none of them very modern Order of succession in metamorphic 

 rocks (p. 299.) Uniformity of mineral character Why the metamorphic 

 strata are less calcareous than the fossiliferous (p. 301.) 



