CONTENTS. XV 



trachyte and granite — Granite reins in Glen Tilt, Cornwall, the Valorsine, and 

 other countries — Different composition of reins from main body of granite — 

 Metalliferous veins in strata near their junction with granite — Apparent isolation 

 of nodules of granite — Quartz veins — "Whether plutonic rocks are ever overlying 

 — Their exposure at the surface due to denudation, - - page *702 



Chapter XXXIV. — 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 composition — Test 

 by included fragments — Recent and Pliocene plutonic rocks, why invisible — Ter- 

 tiary plutonic rocks in the Andes — Granite altering Cretaceous rocks — Granite 

 altering Lias in the Alps and in Skye — Granite of Dartmoor altering Carbonifer- 

 ous strata — Granite of the Old Red Sandstone period — Syenite altering Silurian 

 strata in Norway — Blending of the same with gneiss — Host ancient plutonic 

 rocks — Granite protruded in a solid form — On the probable age of the granites 

 of Arran, in Scotland, - - - - - - - 717 



Chapter XXXV. — Metamorphic Rocks. 



General character of metamorphic rocks — Gneiss — Hornblende-schist — Mica-schist — 

 Clay-slate — Quartzite — Chlorite-sehist — Metamorphic limestone — Alphabetical list 

 and explanation of the more abundant rocks of this family — Origin of the meta- 

 morphic strata — Their stratification — Fossiliferous strata near intrusive masses 

 of granite converted into rocks identical with different members of the metamor- 

 phic series — Arguments hence derived as to the nature of plutonic action — Time 

 may enable this action to pervade denser masses — From what kinds of sedi- 

 mentary rock each variety of the metamorphic class may be derived — Certain 

 objections to the metamorphic theory considered — Partial conversion of Eocene 

 slate into gneiss, - - - - - - - -732 



Chapter XXXVI. — Metamorphic Rods, continued. 



Definition of joints, slaty cleavage, and foliation — Supposed causes of these struc- 

 tures — Mechanical theory of cleavage — Condensation and elongation of slate 

 rocks by lateral pressure — Supposed combination of crystalline and mechanical 

 forces — Lamination of some volcanic rocks due to motion — Whether the folia- 

 tion of the crystalline schists be usually parallel with the original planes of strati- 

 fication — Examples in Norway and Scotland — Foliation in homogeneous rocks 

 may coincide with planes of cleavage, and in uncleaved rocks with those of 

 stratification — Causes of irregularity in the planes of foliation, - - 746 



Chapter XXXVLT. — 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, ......... 758 



