Geology: The Science of the Earth's Crust

by William J. Miller

WE are more or less familiar with the division of all materials of nature into the animal, vegetable, and mineral kingdoms. With slight exceptions minerals are the materials which make up the known part of the earth. In a very real sense, then, mineralogy is the most fundamental of the various branches of the great science of geology because the events of earth history, as interpreted by the geologist, are recorded in the mineral matter (including most rocks) of the earth. When we examine the rocky material or mineral matter of the earth in any region we find that it consists of various kinds of substances each of which may be recognized by certain characteristics. Each definite substance (barring those of organic origin) is called a mineral. Or, more specifically, a mineral is a natural, inorganic, homogeneous substance of definite chemical composition. According to this definition a mineral must be found ready made in nature, must not be a product of life, must be of the same nature throughout, and its composition must be so definite that it can be expressed by a chemical formula. All artificial substances, such as laboratory and furnace products, are excluded from the category of minerals. Coal is not a mineral because it is both organic and of indefinite composition. A few examples of very common substances which perfectly satisfy the definition of a mineral are quartz, feldspar, mica, calcite, and magnetite. Only two substances—water and mercury—are ordinarily liquid minerals. There are nearly a thousand distinct mineral species, and to them and their varieties several thousand names have been applied.

It is a surprising fact that of the eighty or more chemical elements, that is substances which cannot be subdivided into simpler ones, only eight make up more than 98 per cent of the weight of the crust of the earth, though, with one very slight exception, none of the eight exist as such in mineral form. The eight elements are oxygen (nearly 50 per cent), silicon (over 25 per cent), aluminum (over 7 per cent), iron (over 5 per cent), calcium (or “lime”), magnesium (or “magnesia”), sodium (or “soda”), and potassium (or “potash”).

Certain rock formations are made up essentially of but one mineral in the form of numerous grains as, for example, limestone, which consists of calcite (carbonate of lime). Most of the ordinary rocks are, however, made up of two or more minerals mechanically bound together. Thus, in a specimen of granite on the author’s desk several distinct mineral substances are distinguishable by the naked eye. These mineral grains are from one to five millimeters across. Most common among them are hard, clear, glassy grains called quartz; nearly white, hard grains, with smooth faces, called feldspar; small, silvery white plates, easily separable into very thin flakes, called mica; and small, hard, black grains, called magnetite. It is the business of the mineralogist to learn the characters of each mineral, how they may be distinguished from each other, how they may be classified, how they are found in nature, and what economic value they may have. It is an important part of the business of the geologist to learn what individual minerals combine to form the many kinds of rocks, how such rocks originate, what changes they have undergone, and what geological history they record. It is thus clear that the great science of geology is much broader in its scope than mineralogy.

One of the most remarkable facts about minerals is that most of them by far have a crystalline structure, that is they are built up of tiny particles known as molecules. Such crystalline minerals are often more or less regular solid forms bounded by plane faces and sharp angles, such forms being known as “crystals.” How do crystals develop such regularity of form? Any solid is considered to be made up of many very tiny (submicroscopic) molecules held together by an attractive force called cohesion. In liquids the molecules may more or less freely roll over each other, thus altering the shape of the mass without disrupting it. In gases the molecules are considered to be relatively long distances apart and moving rapidly. During the process of change of a substance from the condition of a liquid or gas to that of a solid, due to lowering of temperature or evaporation, the cohesive force pulls the particles (molecules) together into a rigid mass. Under favorable conditions such a solid has a regular polyhedral form. "This results from the fact that the particles or molecules of the substance which, while it was liquid or gaseous, rolled about on one another, have been in some way arranged, grouped and built up. To illustrate this, suppose a quantity of small shot to be poured into a glass: the shot will represent the molecules of a substance in the liquid state, as for example a solution of alum. If, now, we suppose these same shot to be coated with varnish or glue so that they will adhere to each other, and imagine them grouped as shown in Figure 70a, they will represent the arrangement of the molecules of the alum after it has become solid or crystallized. This arranging, grouping, and piling up of molecules is called crystallization, and the solid formed in this way is called a crystal. Figures 70b and 70c show the shot arranged to reproduce two common forms of crystals (e.g., fluorite and calcite)." (Whitlock.)

Fig. 70.—Piles of shot arranged to give some idea of the manner in which molecules are bound together in various crystal forms. (After Whitlock, New York Museum.)

A combination of certain facts regarding crystals furnish all but absolute proof of some sort of regularity of arrangement of particles within them. Among such facts are the following: (1) the wonderful regularity of arrangement of faces upon crystals is practically impossible to account for except as the outward manifestation of regularity of structure or systematic network arrangement of the interior; (2) most crystals split or cleave more or less perfectly in one or more directions presumably in accordance with certain layered structure of the constituent particles; (3) all of the many known forms of crystals can be accurately grouped in regard to their effects upon the passage of light (especially polarized light) through them, each kind or type of network structure presumably producing a different effect upon light; and (4) X-ray photographs have proved that particles, or at least groups of particles, are very systematically arranged within crystals.

It will be instructive for us to make a comparison between the growth of crystals and organisms. Both really grow, but each species of organism is rather definitely limited in size while there is no known limit to the size which may be attained by a crystal so long as material is supplied to it under proper conditions. As a matter of fact crystals vary in size from microscopic to several feet in length, those less than an inch in length being most abundant by far. Organisms mostly grow from within, while crystals grow from material externally added. It is an astonishing fact that in crystals as well as organisms growth takes most rapidly on a wound or broken place. Thus if a crystal is removed from the solution in which it is growing and put back after a corner has been broken off, the fractured surface will build up more rapidly than the rest. Finally, crystals are not necessarily limited in age like organisms. Under certain natural conditions, as, for example, weathering, crystals may decay or be broken up; but where they are protected as constituent parts of rock formations well below the earth’s surface they may remain unchanged for indefinite millions of years. Thus in a ledge of the most ancient known or Archeozoic rock only recently laid bare by erosion one may see crystals which are precisely as they were when they crystallized many millions of years ago.