Part 5 (1/2)
CHAPTER XXIX.
VOLCANIC ROCKS--_continued_.
Trap dike--sometimes project--sometimes leave fissures vacant by decomposition--Branches and veins of trap--Dikes more crystalline in the centre--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 c.o.ke--Inequality in the modifying influence of dikes--Trap interposed between strata--Columnar and globular structure--Relation of trappean rocks to the products of active volcanos--Submarine lava and ejected matter correspond generally to ancient trap--Structure and physical features of Palma and some other extinct volcanos 378
CHAPTER x.x.x.
ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.
Tests of relative age of volcanic rocks--Test by superposition and intrusion--Dike of Quarrington Hill, Durham--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 Pliocene period--Val di Noto in Sicily 397
CHAPTER x.x.xI.
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--Ravines and ancient gravel-beds--Jets of air called Bufadors--Age of the Catalonian volcanos--Miocene period--Brown-coal of the Eifel and contemporaneous trachytic breccias--Age of the brown-coal--Peculiar characters of the volcanos of the upper and lower Eifel--Lake craters--Tra.s.s--Hungarian volcanos 408
CHAPTER x.x.xII.
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--River dammed up by lava-current--Range of minor cones from Auvergne to the Vivarais--Monts Dome--Puy de Come--Puy de Pariou--Cones not denuded by general flood--Velay--Bones of quadrupeds buried in scoriae--Cantal--Eocene volcanic rocks--Tuffs near Clermont--Hill of Gergovia--Trap of Cretaceous period--Oolitic period--New Red Sandstone period--Carboniferous period--Old Red Sandstone period--”Rock and Spindle” near St. Andrews--Silurian period--Cambrian volcanic rocks 422
CHAPTER x.x.xIII.
PLUTONIC ROCKS--GRANITE.
General aspect of granite--Decomposing into spherical ma.s.ses--Rude columnar structure--a.n.a.logy and difference of volcanic and plutonic formations--Minerals in granite, and their arrangement--Graphic and porphyritic granite--Mutual penetration of crystals of quartz and felspar--Occasional minerals--Syenite--Syenitic, talcose, and schorly granites--Eurite--Pa.s.sage of granite into trap--Examples near Christiania and in Aberdeens.h.i.+re--a.n.a.logy in composition of trachyte and granite--Granite veins in Glen Tilt, Cornwall, the Valorsine, and other countries--Different composition of veins 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 436
CHAPTER x.x.xIV.
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--Tertiary plutonic rocks in the Andes--Granite altering Cretaceous rocks--Granite altering Lias in the Alps and in Skye--Granite of Dartmoor altering Carboniferous strata--Granite of the Old Red Sandstone period--Syenite altering Silurian strata in Norway--Blending of the same with gneiss--Most ancient plutonic rocks--Granite protruded in a solid form--On the probable age of the granites of Arran, in Scotland 449
CHAPTER x.x.xV.
METAMORPHIC ROCKS.
General character of metamorphic rocks--Gneiss--Hornblende-schist --Mica-schist--Clay-slate--Quartzite--Chlorite-schist--Metamorphic limestone--Alphabetical list and explanation of other rocks of this family--Origin of the metamorphic strata--Their stratification is real and distinct from cleavage--Joints and slaty cleavage--Supposed causes of these structures--how far connected with crystalline action 463
CHAPTER x.x.xVI.
METAMORPHIC ROCKS--_continued_.