The Science of Rocks: Common Rock Types

Everywhere beneath the mantle of soil and vegetation that covers the surface of the land lies rock, the solid platform upon which the superficial soils and organic debris of earth rests. Here and there on mountain tops, in cliffs and ledges, we see this underlying rock exposed and projecting out from the soil: we know that it must also underlie the sea in the same way. The outer shell of the earth then is made of rock, which forms the foundation upon which rest all the surface things with which we are acquainted. How thick this zone of rock is we cannot easily see, but upon it we live and exert our activities; into it we penetrate for coal, oil, gas, metal ores, gemstones and other useful things upon which the material features of our modern technological civilization depend. The rocks of the earth's crust is therefore of the highest importance to prospectors, and the information which we have acquired concerning it, by examination and study, forms a valuable branch of human knowledge. Our knowledge of all the various things which together make up that part of the earth which it is permitted us to examine and study and which has been comprehended under the heading of Geology has now increased to such a degree that this science has split up into a number of well-defined, subordinate branches or geological sciences. Thus Meteorology is the science of the atmosphere, the summation of our knowledge of the causes and movements of winds, storms, rain, the distribution of heat and cold, and in general the study of the various factors that affect the air and its movements and of the laws that govern them.
Physiography takes account of the surface features of the earth, of the distribution of land and water and of the agencies which are modifying them, the effects of climates and the various 'causes which together produce the topography which the earth's surface now exhibits. Paleontology is the science resulting from the study of the remains of past life upon the earth, as shown by the fossils enclosed in the rocks, and teaches not only the different forms which have existed but also seeks to discover the various movements or migrations of life upon the earth in past ages.



The Common Rock Types:
I. Igneous Rocks: General Concepts
The Granite Family
The Diorite Family
The Gabbro Family
The Diabase (Dolerite) Family
The Peridotite Family
The Serpentine Group
The Rhyolite Family
The Andesite Family
The Basalt Family
Pegmatite Dikes and Veins

II. Sedimentary Rocks, general concepts
Sandstone and Related Rocks
Shale and Related Rocks
Limestone and Other Carbonate Rocks
Iron Ore Sedimentary Rocks

III. Metamorphic Rocks - General Concepts
Gneiss Rocks
Mica Schist and Related Schist Rocks
Quartzite Rock
Slate Or Argillite
Greenstone / Chlorite-Schist / Greenschist

Petrology, in the same way, has now become a separate branch, one of the geologic sciences. It comprises our knowledge of the rocks forming the crust of the earth, the results of our studies of the various component materials which form them, of the different factors and the laws governing them which have led to their formation, and of their behavior under the action of the agencies to which they have been subjected, and endeavors to classify the kinds into orderly arrangement.
The terms Petrology and Petrography are not absolute synonyms though often so used in a general way. The former has been denned above; the latter more particularly refers to the description of rocks and especially with respect to their study by means of the microscope as explained later thus microscopic petrography. Petrology is used for the science in its broader aspects as well and covers the geological and chemical relations of rocks: thus strictly denned petrography may be said to be a branch of petrology. The synonym Lithology has become nearly obsolete. Petrology means the science of rocks; lithology would be the science of stones, but the word stone is now used in a generalized and popular way for architectural or commercial purposes and also to designate any loose piece of rock of unknown origin, rendering the term non-specific for scientific use.



By the term "rock," geologically speaking is meant the material composing one of the individual parts of the earth's solid crust, which, if not exposed, everywhere underlies the superficial covering of soil, vegetation or water which lies upon it. The popular understanding of this term, that it denotes a hard or firm substance, is not, geologically, a necessary one, for a soft bed of clay or of volcanic ash is as truly a rock as a mass of the hardest granite. Moreover it implies within limits, which will be explained elsewhere, a certain constancy of chemical and mineral composition of the mass recognized as forming a particular kind of rock. Thus the chance filling of a mineral vein by variable amounts of quartz, calcite and ores is not accepted by petrographers as forming a definite kind of rock. The term is also used with different meanings; it may be denote of the substance forming parts of the earth's crust, as quartz and feldspar arranged in a particular manner are said to form a rock like granite or it may refer to the masses themselves and thus possess a larger, geological significance. In a general way the former may be said to be a petrographic, the latter a geologic usage. When used in this broader geologic sense the mass recognized as an individual kind of rock must possess definite boundaries and show by its relations to other rock masses that it owes its existence to a definite geological process. The absolute size of the mass is not involved in this, for a seam or dike of granite cutting rocks of other kinds may be as thin as cardboard or a mile in thickness.
Rocks are sometimes defined as aggregates of one or more minerals, but this is not a broad enough or wholly correct definition. Rocks may be composed entirely of minerals or entirely of glass or of a mixture of both. Minerals are substances having definite chemical compositions and usually of crystalline structure; glasses are molten masses chilled and solidified without definite composition and structure. Rocks composed wholly of minerals may be simple or compound, that is, the rock may be formed of one kind of mineral alone, as for example, some of the purest marbles which consist of calcite only or of a mixture of two or more like ordinary granite which is made of grains of quartz, feldspar and mica.

According to their mode of origin and the position of the masses with respect to the earth's crust and to each other, rocks naturally divide themselves into three main groups, divisions which are recognized by practically all geologists. These are the igneous rocks made by the solidification of molten material; the sedimentary or bedded rocks formed by the precipitation of sediments in water, to which may be added the small group of aeolian or wind formed deposits, and the metamorphic rocks, those produced by the secondary action of certain processes such as heat or pressure upon either igneous or sedimentary ones by which their original characters are wholly or partly obscured and replaced by new ones and which are therefore most conveniently considered in a separate group. This grouping will be used in this work, and each group with its further subdivisions, their characters, relations, etc., will be treated by itself. Summarizing then what has just been stated, we have:

Igneous Rocks, solidified molten masses.

Sedimentary Rocks, precipitated sediments.

Metamorphic Rocks, formed by heat or pressure on other rocks.

We will take a look at each of these three major classes of rock and the most important members of each class individually.



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