CONTENTS. XV 



Chapter XXX. — On the Different Ages of the Volcanic Rocks. 



Tests of relative age of volcanic rocks — Test by superposition and intrusion — 

 Test by alteration of rocks in contact — Test by organic remains — Test of age 

 by mineral character — Test by included fragments — Volcanic rocks of the Post- 

 Pliocene period — Basalt of Bay of Trezza in Sicily — Post-Pliocene volcanic 

 rocks near Naples — Dikes of Somma — Igneous formations of the Newer Plio- 

 cene period — Yal di Noto in Sicily .... Page 519 



Chapter XXXL — On the Different Ages of the Volcanic Rocks — continued. 



Volcanic rocks of the Older Pliocene period — Tuscany — Rome — Volcanic region 

 of Olot in Catalonia — Cones and lava-currents — Miocene period — Brown-coal 

 of the Eifel and contemporaneous trachytic rocks — Age of the brown-coal — 

 Peculiar characters of the volcanoes of the Upper and Lower Eifel — Lake craters 

 — Trass — Hungarian volcanoes ------ 530 



Chapter XXXII. — On the Different Ages of the Volcanic Rocks — continued. 



Volcanic rocks of the Pliocene and Miocene periods continued — Auvergne — Mont 

 Dor — Breccias and alluviums of Mont Perrier, with bones of quadrupeds — 

 Mont Dome — Cones not denuded by general flood — Velay — Bones of quadru- 

 peds buried in "scoriae — Cantal — Eocene volcanic rocks — Tuffs near Clermont — 

 Hill of Gergovia — Trap of Cretaceous period — Oolitic period — New Red Sand- 

 stone period — Carboniferous period — Old Red Sandstone period — Silurian pe- 

 riod — Cambrian volcanic rocks ------ 545 



Chapter XXXIII. — Plutonic Rocks — Granite. 



General aspect of granite — Analogy and difference of volcanic and plutonic for- 

 mations — Minerals in granite — Mutual penetration of crystals of quartz and 

 felspar — Syenitic, talcose, and schorly granites — Eurite — Passage of granite 

 into trap — Granite veins in Glen Tilt, and other countries — Composition of 

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

 Quartz veins — Whether plutonic rocks are ever overlying — Their exposure 

 at the surface due to denudation ------ 560 



Chapter XXXIV. — On the different Ages of the Plutonic Rocks. 



Difficulty in ascertaining the age of a plutonic rock — Test of age by relative posi- 

 tion — 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 — Granite altering Carboniferous strata — Granite of the Old Red 

 Sandstone period — Syenite altering Silurian strata in Norway — Oldest plutonic 

 rocks — Granite protruded in a solid form — Age of the granites of Arran, in 

 Scotland --------- 573 



Chapter XXXV. — Metamorphic Rocks. 



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

 schist — Clay-slate — Quartzite — Chlorite-schist — Metamorphic limestone — Al- 

 phabetical list and explanation of the more abundant rocks of this family — 

 Origin of the metamorphic strata — Their stratification — Fossiliferous strata 

 near intrusive masses of granite converted into different members of the meta- 

 morphic series — Objections to the metamorphic theory considered — Partial 

 conversion of Eocene slate into gneiss - - - - - 587 



Chapter XXXVI. — Metamorphic Rocks — continued. 



Origin of the metamorphic rocks, continued— Definition of joints, slaty cleavage, 

 and foliation — Causes of these structures — Mechanical theory of cleavage — - 

 Supposed combination of crystalline and mechanical forces — Lamination of 

 some volcanic rocks due to motion — Whether the foliation of the crystalline 

 schists be usually parallel with the original planes of stratification - 600 



