CLASSIFICATION OF ROCKS AND GEOLOGICAL THE FORMATIONS HE earth is made up of rocks, the term including not only the hard, solid matter of the earth but also all loose substances sand, gravel, clay, soil, etc. Rocks are composed of mineral or aggregates of minerals, and have been classified in many different ways. The following is one of the most useful of the primary classifications: 1. Igneous. 2. Aqueous and Eolian (Sedimentary). 3. Metamorphic. The igneous rocks are probably the most important group in relation to metal mining and are classified according to the minerals of which they are composed and their texture. The latter depends chiefly on the size, relative and absolute, of the crystals which go to make up the rock. One of the best and most practical classifications of the igneous rocks I have seen is given in tabular form herewith.1 The table is a genuine multum in parvo of petrography, giving the names, the mineralogical and chemical compositions and texture of practically all the igneous rocks, both common and rare, met with in scientific literature. IGNEOUS ROCKS EXPLANATION OF TABLE Igneous rocks are those which have consolidated from a fused or semi-fused condition. They may be best classified by means of their mineralogical composition, and partly by their mode of occurrence. By the latter method they fall into two great classes: the extrusives, or those thrown out upon the surface by volcanic action, etc., and the intrusives, or those thrust up into the earth's crust but which do not reach the surface except by sub 1 This and the accompanying explanation of the table are kindly furnished by Dr. F. J. Pack, professor of geology, Brigham Young College, Logan, Utah. sequent erosion. Among the forms thus produced are the socalled batholiths, laccoliths, bosses, sheets, dikes, etc. The textures of the two classes differ greatly. As a rule the extrusive rocks are fine-grained or felsitic, while the intrusive rocks are coarse-grained, as shown in the granites and porphyries.? It is surprising to note the small number of minerals present in igneous rocks. Half a score will include all the more common ones, and a score almost all of them. Among the ones commonly present are the following: quartz, the feldspars, the micas, hornblende, augite, and the iron ores. The feldspars are divided into the orthoclase and plagioclase series. Some one of the feldspars is present in practically every rock. It is convenient, therefore, that the genetic classification be based upon the feldspar content. In the third column of the accompanying table it will be noted that the chief feldspar of the first six groups is orthoclase, while of the remaining six it is plagioclase. The other essential minerals of each group are indicated in the same column. It may be seen, therefore, that a rhyolite is composed of orthoclase, quartz, and one or more of the following: biotite, hornblende, augite. Furthermore, a dacite is composed of plagioclase, quartz, and one or more of the following: biotite, hornblende, augite. The only difference between a rhyolite and a dacite is that the feldspar in the former is orthoclase, while in the latter it is plagioclase. In addition to the essential minerals a number of accessory ones not infrequently occur. A list of such is set opposite each group. At the rhyolite-granite end of the series the light-colored 2 The term "porphyry" is one so often used, especially among practical miners, that a word of explanation of the name may be useful. It was originally applied to any rock consisting of large crystals imbedded in a fine-grained, or felsitic, ground mass. When rocks were more carefully studied and accurately classified by the aid of the petrographic microscope, it was found that the rocks called porphyry were of varying mineralogical composition corresponding to the classes of the coarsely crystalline (holocrystalline) rocks such as the granites, gabbros, etc. Consequently the word "porphyritic" has come to be used by scientific petrographers as the name of a texture - made up of large well-formed crystals of any rock mineral imbedded in a fine-grained ground mass, which usually consists of crystals too small to be distinguished by the naked eye. By prefixing the name of the class of holocrystalline rocks of corresponding mineralogical composition, granite-porphyry, dioriteporphyry etc., accurate names are furnished for the rocks. For field work, however, the term "por phyry" is allowable and is very useful as a provisional name until the nature of the rock is more accurately determined by the microscope, but it is not used scientifically for any rock of definite mineralogical composition. Another textural term is "granitoid" which is applied to any rock composed of crystals, all of which are large enough to be distinguished by the naked eye. The term "granite," however, is also used by scientific petrographers as the name of a rock of definite mineralogical composition, as shown in the table. minerals are in excess, and, therefore, these rocks are usually light colored. At the basalt-gabbro end the dark-colored minerals predominate; these rocks are, therefore, much darker than the ones at the opposite end. This fact is noted in the fifth column. It has already been stated that the textures of the various igneous rocks differ greatly. This is brought about not by mineralogical composition, but by the conditions under which cooling occurred. Two rocks such as granite and rhyolite differ but little in composition, but greatly in general appearance. The rhyolite is an extrusive rock; the granite an intrusive. The former was cooled quickly; the latter, slowly. The texture of the various classes is indicated under the proper heading. From a great many analyses of igneous rocks the data under the head of "Chemical Composition" have been selected. The maximum, minimum, and average range of each of the compounds present is given. For example, in analyzing rhyolites the maximum amount of silica obtained from any specimen was 83.59 per cent., the minimum 63.63 per cent., and in most specimens it ranged from 65 per cent. to 75 per cent. = Under the heading "Related Groups" the connection between the various classes may be easily traced. The signs and + indicate respectively the absence or presence of the mineral following in connection with the type rock named in column 2. The braces include all the added or subtracted minerals necessary to make up the new rock named after the sign. Where one mineral stands alone with sign before it, this means that only this mineral is added or subtracted. If from a given rock a mineral is introduced or taken out, the resultant rock belongs to a different class. Example: If from a rhyolite we take the quartz, the rock is then a trachyte. Or if to a trachyte we add quartz, we get a rhyolite. This means that the only difference between a rhyolite and a trachyte is that the former contains quartz and the latter does not. Again, if from a rhyolite we take the orthoclase and add plagioclase, the result is a dacite. Or if from a dacite we take the plagioclase and add orthoclase, the result is rhyolite. The difference between these two rocks is, therefore, that the rhyolite contains orthoclase and the dacite plagioclase. Still further, if quartz be taken from a granite, the result is a syenite. Or if quartz be added to a syenite, the result is a granite, The relation between any of the various groups may be easily traced by means of the data in this column. At the extreme right of the table a number of varieties and intermediate groups are arranged, and their relation to the chief subdivisions indicated. These are of much less frequent occurrence in nature than the great classes named in the second column. AQUEOUS AND EOLIAN ROCKS The aqueous and eolian rocks are those which owe their present form to the action of water and wind respectively, being composed of broken-down igneous or previous sedimentary rocks, or of corals, shells and other organic remains, of precipitates from solution, etc. Those of aqueous origin are commonly called sedimentary rocks and are most frequently subdivided according to composition, as limestones, sandstones, shales, etc. FIG. 101.-Illustrating terms used in describing the From Stretch; Prospecting, Locating and Valuing Mines. The sedimentary rocks have been deposited throughout all the ages since air and water, wind and wave have been acting on the solid rock surface of the earth, being laid down in a succession of beds. One of the great achievements of geology is that it has worked out the succession of the sedimentary beds from inconceivably remote times. It has ascertained that this succession is, in a general way, the same all over the earth, and the different geological ages have received names that are of general application. The following gives one of the late arrangements of the periods |