phism has often proceeded further, and has absolutely altered the character of these rocks, so that the original compact sandstone or shale has been converted into a rock totally different from its original character or that of the rocks which we know by these names. Such rocks, in a general way, are known as metamorphic or crystalline rocks. Among such as were originally in the condition of mud or sand, and subsequently, through consolidation, shale beds and sandstones, by further metamorphism we find many in the present form of slates, quartzites, gneisses, and gneissoid rocks in general, micaceous, talcose, and other schists, some so-called porphyries, some kinds of granite, etc. It is important to remember this fact, because many valuable ore deposits are found in the very old rocks of this description. They are usually, and sometimes rather loosely, referred to as Primitive Rocks, Archæan and Algonkian Rocks, Crystalline Rocks, Metamorphic Rocks, etc.1

Highly folded and crystalline Primitive Rocks (Logan).

Unconformity or later Sediments deposited on Eroded Surfaces of Upturned Strata (Le Conte).

Upturned and Eroded Strata in Colorado (Hayden).

1 It must not be thought from this that all gneissoid and similar rocks are

necessarily of sedimentary origin, for it is extremely probable that many of them

The third sedimentary rock (limestone) cannot be metamorphosed, generally speaking, further than marble, which is geologically termed crystalline limestone. Rarely, however, it has been converted into the rock known as serpentine.1 Limestone, whether uncrystalline or crystalline, in this country, however, usually when in the former condition, is a most important repository of many valuable metallic ores, its calcareous nature having, in some way not always thoroughly understood, exerted a very favorable influence upon the deposition of mineral matter, especially of certain kinds. Much of this mineral matter, as will be seen in the latter part of this geological preface, was probably contained in hot waters which have come up through or in juxtaposition to the limestone strata.

SECOND. The second great class of rocks are those which are known as Eruptive Rocks, and which have all, at one time or another, been in a molten (semi-fluid or fluid) condition. The material from which these rocks have been formed came up from unknown depths or melted areas in the earth, either through the vents of volcanoes or more frequently through great or small cracks or breaks in the rocky crust above, and was poured out over the then surface of the earth. Sometimes, however, it has not reached the surface, but after coming up a certain distance was unable to ascend further, and subsequently slowly solidified far beneath the surface. In one form or another these rocks probably form a large portion of the earth's interior, and underlie all stratified rocks, but at varying depths. It is not at all certain that they form the central mass of the earth, and they are by some supposed to occupy, generally speaking, a position between the outer crust and the interior central mass, whatever the nature of the latter may be.

These eruptive rocks are of many kinds, owing to their different chemical composition. These are, in a general way, all of the granites (except some of the metamorphic granites above referred to), syenites, and porphyries, as well as the rocks which

are very ancient and greatly metamorphosed eruptives. The schistose structure of many of these rocks has probably been given to them by subsequent heating and great pressure. The researches of the Canadian Geological Survey have thrown much light upon the probable origin of

many of these very old rocks, and some of the best Canadian geologists are disposed to doubt the original sedimentary character of many schists and kindred rocks.

1 Most serpentines, however, are changed eruptive rocks known as peridotites.

are known as diorite, trap, diabase, basalt, phonolite, rhyolite, trachyte, andesite, gabbro, obsidian, pumice, etc.

In mining, one usually meets with these rocks either as lava flows, when they form great beds often extending over large and usually very irregular areas, or as dikes which have welled up from below through crevices or cracks in the pre-existing and overlying rocks. These pre-existing rocks may have been either sedimentary or eruptive. This is merely a matter of accident, according to the character of the rocks at the localities where the volcanic agencies to which the eruptives owe their origin have been acting. Thus we often find that dikes of trap or some other eruptive rock have come up through coal measures, and through the coal beds themselves. Often ore deposits, which in many cases had been formed before the injection of the molten material, are found to be cut in two by a so-called dike. At other times these eruptive rocks have come up in immense masses, pushing aside the strata above or extending their liquid. tongues far out between the stratified rocks, usually along the lines of stratification or at a very small angle with them. They often appear to be stratified when this has occurred, and especially is this true when they have been poured out over the bed of the sea in which sediments were being deposited and afterward covered with sedimentary material. Sometimes we meet with a bed or flow of lava containing innumerable angular fragments of eruptive rocks cemented together by the lava. Some of these were very probably thrown out of an old crater, and in falling became embraced in the lava flow; or more often these angular pieces simply represent the unfused portions of the igneous rock of which the lava represents the fused portion, the two having been brought up together through the volcanic vent. It sometimes happens that angular rubble has been picked up by a lava flow. These rocks are called volcanic breccia.

These eruptive rocks, especially when in the form of dikes, should be studied very carefully, because of the important part which they have played in the crigin of deposits of the valuable metals or of their ores; for in the great majority of cases where deposits of these metals are found, eruptive rocks of one character or another are apt to be found traversing the neighboring country rock and often in the immediate vicinity, although the converse of this proposition is far from being true. It can be proven in a great

Dikes of one kind of Eruptive Rock cutting through mountain mass of another kind of Eruptive Rock (Geikie).

many cases, not only that the region which became mineralized was very hot during this process, but that the dikes or eruptive rocks came up at approximately the same time that the ore deposits were formed. The intimate connection between the two is thus established. The formation of the dikes is usually supposed and is often known to have preceded the mineralization; that is, the material from which they are formed was in the majority of cases injected before the latter process was begun. These dikes vary from a foot or less in thickness to thousands of feet, and may be traced across the country only a short distance or for many miles. Sometimes mountain masses are made up of dikes of one sort or another, or several kinds in juxtaposition or intersecting each other. Those which are commonly recognized as dikes, however, are usually not very thick, being from one up to somewhat over one hundred feet.

Eruptive rocks can usually be distinguished with great ease, not only from the position they occupy and the absence of the lines of sedimentation, but also by their general appearance, from any of the three stratified or sedimentary rocks above referred to; namely, sandstones, shales, and limestones. Sometimes, however, as in the case of some of the metamorphic rocks of both classes, it becomes difficult to do this. In such a case it is necessary to send a specimen of the rock to some accredited petrographer so that he may examine a section of it under the microscope. In this way its original character can usually be determined without difficulty.

Rock Folding and Faulting. It must be remembered that all these rocks, both sedimentary and eruptive, are frequently found to occupy positions now which are quite different from those originally occupied by them. Thus, consolidated sediments of thousands of feet in thickness may have lain for perhaps millions of years undisturbed in a nearly horizontal position, when, owing probably to the slow cooling and consequent irregular contraction of the interior of the earth, the so-called crust is thrust upon itself with an inconceivable force. By means of this pressure applied horizontally, or approximately so, nearly level strata are bent and twisted along certain lines of weakness in every conceivable manner, though they may have been the very hardest kind of rock. Of course all rocks, sedimentary and eruptive, composing the socalled crust are involved in this folding or crushing, but it is more easily discerned in the case of the former than in the latter. canic action sometimes produces somewhat similar results; but when it has been the cause of the tilting of rocks, the disturbance has usually been regional, and very great areas have not been involved, as has been the case where the former agency has been operative.

Vol

Section of Appalachian Range showing folded strata (Rogers).

Accompanying this folding or crumpling, but sometimes without it, as in the case of certain earth tremors produced by volcanic agencies, cracks or fissures have been established which reach to unfathomable depths, and the whole mass of rock on one side of