CONTENTS. 



vii 



CHAPTER VIII. 



VOLCANIC ROCKS — continued. 



Trap dikes — sometimes project — sometimes leave fissures vacant by decom- 

 position — Branches and veins of trap — Dikes more crystalline in the centre 

 (p. 105.) — Foreign fragments of rock imbedded — Strata altered at or near 

 the contact — Obliteration of organic remains — Conversion of chalk into 

 marble — and of coal into coke (p. 108.) — Inequality in the modifying influ- 

 ence of dikes — Trap interposed betv^^een strata — Columnar and globular 

 structure (p. 110.) — Relation of trappean rocks to the products of active 

 volcanos (p. 114.) — Submarine lava and ejected matter corresponds gene- 

 rally to ancient trap. 



CHAPTER IX. 



PLUTONIC ROCKS — GRANITE. 



General aspect of granite — Decomposing into spherical masses — Rude colum- 

 nar structure — Analogy and difference of volcanic and plutonic formations 



— Minerals in granite, and their arrangement — i&raphic and porphyritic 

 granite (p. 121.) — Occasional minerals — Syenite — Syenitic, talcose, and 

 schorly granites — Eurite — Passage of granite into trap — Examples near 

 Christiania and in Aberdeenshire — Analogy in composition of trachyte and 

 granite — Granite veins in Glen Tilt, Cornwall, the Valorsine, and other 

 countries (p. 123.) — Different composition of veins from main body of granite 



— Metalliferous veins in strata near their junction with granite (p. 128.) — 

 Apparent isolation of nodules of granite — Quartz veins — Whether plutonic 

 rocks are ever overlying — Their exposure at the surface due to denudation 

 (p. 131.) 



CHAPTER X. 



METAMORPHIC ROCKS. 



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

 schist — Clay-slate (p. 133.) — Quartzite — Chlorite-schist — Metamorphic 

 limestone — Alphabetical list and explanation of other rocks of this family 



— Origin of the metamorphic strata (p. 135.) — Their stratification is real 

 and distinct from cleavage — On joints and slaty cleavage (p. 138.) — Sup- 

 posed causes of these structures — how far connected with crystalline action. 



CHAPTER XI. 

 METAMORPHIC ROCKS — Continued, 



Strata near some intrusive masses of granite converted into rocks identical 

 with different members of the metamorphic series — Arguments hence de- 

 rived as to the nature of plutonic action (p. 146.) — Time may enable this 

 action to pervade denser masses — From what kinds of sedimentary rock 

 each variety of the metamorphic class may be derived (p. 150.) —• Certain 

 objections to the metamorphic theory considered. 



