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Read Ebook: Geology by Geikie James
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PAGE INTRODUCTORY 7
CLASSIFICATION OF ROCKS 8
MINERALOGY 12 ROCK-FORMING MINERALS 14
PETROLOGY-- MECHANICALLY FORMED ROCKS 17 CHEMICALLY FORMED ROCKS 19 ORGANICALLY DERIVED ROCKS 20 METAMORPHIC ROCKS 21 IGNEOUS ROCKS 23 STRUCTURE AND ARRANGEMENT OF ROCK-MASSES-- Stratification, &c.; Mud-cracks and Rain-prints; Succession of Strata; Extent of Beds; Sequence of Beds--Joints; Cleavage; Foliation; Concretions; Inclination of Strata; Contemporaneous Erosion; Unconformability; Overlap; Faults; Mode of Occurrence of Metamorphic and Igneous Rocks; Mineral Veins 26-46
DYNAMICAL GEOLOGY-- THE ATMOSPHERE AS A GEOLOGICAL AGENT OF CHANGE 46 WATER AS A GEOLOGICAL AGENT OF CHANGE 48 GEOLOGICAL ACTION OF PLANTS AND ANIMALS 60 SUBTERRANEAN FORCES 64 METAMORPHISM 72
PHYSIOGRAPHY 74
PALAEONTOLOGY 77
HISTORICAL GEOLOGY 84
QUESTIONS 89
INTRODUCTORY.
MINERALOGY.
ROCK-FORMING MINERALS.
It is needless to describe the minerals minutely here. The student can only learn to distinguish the different species by carefully examining actual specimens.
Having now mentioned the chief rock-forming minerals, we proceed to a brief description of some of the more typical representatives of the five great classes of rocks referred to at page 8.
PETROLOGY.
MECHANICALLY FORMED ROCKS.
CHEMICALLY FORMED ROCKS.
ORGANICALLY DERIVED ROCKS.
METAMORPHIC ROCKS.
There are a number of other metamorphic rocks, but those mentioned are the most commonly occurring species.
IGNEOUS ROCKS.
ACIDIC OR FELSPATHIC GROUP.
AUGITIC AND HORNBLENDIC OR BASIC GROUP.
STRUCTURE AND ARRANGEMENT OF ROCK-MASSES.
When strata shew many and rapid curves, they are said to be contorted. The diagram section will best explain what is meant by this kind of structure.
The degree of inclination is very variable. It may occur at almost any angle up to vertical. But, as a rule, the hade of the more powerful faults is steeper than that of minor displacements.
Igneous rocks have also in some cases undergone considerable alteration; fine-grained tuffs, for example, occasionally assume a crystalline texture.
These remarks on the mode of occurrence of igneous rocks are meant to refer chiefly to those masses which occur in regions where volcanic action has long been extinct, as, for instance, in the British Islands. In the sequel, some account will be given of the appearances presented by modern volcanoes and volcanic rocks.
MINERAL VEINS.
DYNAMICAL GEOLOGY.
THE ATMOSPHERE.
WATER.
River-ice is often a potent agent of geological change. Stones get frozen in along the margins of a river, and often d?bris falls down from cliff and scaur upon the surface of the ice; when thaw sets in, and the ice breaks up, stones and rubbish are frequently floated for long distances, and may even be carried out to sea before their support fails them, and they sink to the bottom. In some cases, when the ice is very thick, it may run aground in a river, and confuse and tumble up the deposits gathering at the bottom. Ice sometimes forms upon stones at the bottom of a river, and floats these off; and this curious action may take place even although no ice be forming at the time on the surface of the water.
GEOLOGICAL ACTION OF PLANTS AND ANIMALS.
SUBTERRANEAN FORCES.
METAMORPHISM.
PHYSIOGRAPHY.
PALAEONTOLOGY.
From what has been stated in the foregoing paragraphs, it is clear that in our endeavours to decipher the geological history of our planet, palaeontological must go hand in hand with stratigraphical evidence. We may indeed learn much from the mode of arrangement of the rocks themselves. But the test of superposition does not always avail us. It is often hard, and sometimes quite impossible, to tell from stratigraphical evidence which are the older rocks of a district. In the absence of fossils we must frequently be in doubt. But physical evidence alone will often afford us much and varied information. It will shew us what was land and what sea at some former period; it will indicate to us the sites of ancient igneous action; it will tell us of rivers, and lakes, and seas which have long since passed away. Nay, in some cases, it will even convince us that certain great climatic changes have taken place, by pointing out to us the markings, and d?bris, and wandered blocks which are the sure traces of ice action, whether of glaciers or icebergs. The results obtained by combining physical and palaeontological evidence form what is termed Historical Geology.
HISTORICAL GEOLOGY.
QUESTIONS.
Section 1. What is Geology?
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