Igneous rock

“ The mere antiquity of an igneous rock is a circumstance absolutely inefficient in accounting for any other of its characters than the degree of superficial waste, or internal change by particular agencies; and therefore an inquiry into the composition and structure of such rocks must in the first instance include the whole series of igneous products, if we wish to determine, in the first place, the conditions to which particular phenomena are due, and, finally, to obtain a correct general history of the change of these conditions in the order of geological time. ”

“ A rock is made up of minerals. The igneous rock granite, for example, consists chiefly of quartz and feldspar; other minerals such as mica and hornblende are commonly present.
9 IGNEOUS ROCKS
Igneous rocks result from the cooling of hot, molten rock material or magma. Magma that reaches the surface through volcanoes is called lava. Magma comes from deep within the earth and is made up of a mixture of molten mineral materials. Igneous rocks have been forming throughout the geologic past and are still forming today. ”

“ These vast bodies of igneous rock, which may reach hundreds of miles in diameter, differ little from bosses except in their immense bulk. Like bosses, regional intrusions give off dikes and sheets and greatly change the rocks about them by their heat. They are now exposed to view only because of the profound denudation which has removed the upheaved dome of rocks beneath which they slowly cooled. Such intrusions are accompanied —whether as cause or as effect is still hardly known—by deformations, and their masses of igneous rock are thus found as the core of many great mountain ranges. ”

“ Where an igneous rock has been invaded by metamorphic changes, it may be observed to have been first broken down into separate lenticles, the cores of which may still retain, with little or no alteration, the original characteristic minerals and crystalline structure of the rock. ”

“ I believe, therefore, that here, as elsewhere, wherever I have had an opportunity of making accurate observations, the mere intrusion or outburst of igneous rock has had little or no immediate connexion with the dislocations or elevations of the rock. ”

“ The central cores of many of the now deeply eroded ranges consist of granite and other similar rocks, and are surrounded by sedimentary beds which range in age from the oldest stratified rocks now known to the youngest. Igneous rocks in great variety and in all forms incident to an extruded or volcanic and intruded or plutonic origin are present. The many disturbances that have occurred have led to the formation of mineral veins and the impregnation of rock masses with ores of various kinds—such as gold, silver, lead, copper, etc.—which have been mined with great success at many localities. ”

“ Either a sedimentary or an igneous rock, which has been altered by the combined activities of heat, pressure and chemical action, becomes a metamorphic rock. The process is 233 essentially one, during which the layers of rock come under the influence of such temperatures as are associated with the formation of granite or lavas. Such material as is actually melted becomes igneous rock, but adjacent to the masses actually melted are other rocks which do not melt but, according to the temperature, are more or less changed, and these are the metamorphic rocks. ”

“ Molten rock ejected from the interior of the earth and cooling form the igneous rocks we have spoken of. They are easily distinguished from the sedimentary rock, as they have no appearance of stratification and contain no fossils. ”

“ A series of specimens may be collected showing unaltered or at least quite recognizable sedimentry rocks at the one end, and thoroughly crystalline igneous rocks at the other. Thus the remarkable fact is brought home to the mind that ordinary sandstones, shales, and other sedimentary materials may in the course of ages be converted by underground changes into crystalline granite. The framework of the land, besides being knit together by masses of igneous rock intruded from below, has been strengthened by the welding and crystallization of its lowest rocks. ”

“ Masses of igneous rock cannot be intruded within the crust without an accompanying deformation on a scale corresponding to the bulk of the intruded mass. The overlying strata are arched into hills or mountains, or, if the molten material is of great extent, the strata may conceivably be floated upward to the height of a plateau. ”

“ Rounded boulder shaped mountains of granite or igneous rocks are to me far more impressive than much taller ones of other formations. One feels that they are solid, and are welded to the central foundations of earth; that they were the offsprings of primal overpowering heat, while the others are made up of tiny particles of disintegrated igneous stone, loosely thrown together by glacial moraines or dropped at ocean's bottom, and after eons of time compressed into hardness. ”

“ The ideas here expressed will be much better appreciated by the reader after following through the salient points in the history of North America as set forth in the succeeding pages.
Again, the reader may ask, by what line of reasoning do we conclude that these stratified rocks are so exceedingly ancient? All rocks of Archeozoic Age, including strata as well as certain younger igneous rocks (see below) , invariably occupy a basal position in relation to all other rock systems. ”

“ Certain minerals, such as magnetite, ilmenite, spinel, corundum, etc., are often found as primary segregations within the mass of igneous rock. These and other minerals, including minerals of tin and tungsten, monazite, tourmaline, rutile, and various precious stones, are characteristically developed in pegmatites, which are known to be igneous rocks, crystallized in the later stages of igneous [Pg 40] intrusion. ”

“ In all these cases the mass of igneous rock is only discovered when the overlying rocks have been eroded off. The great mass of molten magma under the arches of mountain ranges simply cools slowly into a granitic type of rock. These masses are exposed when the thousands of feet of overlying rock are eroded off. When these masses are exposed, if of but a few miles in extent, they are called stocks, but, if of many miles in length and breadth, they are batholiths, 175 and are very characteristic of the heart of mountain ranges. ”

“ According to this view, the differences between some of the most remarkable igneous rocks are merely in the degrees of arrangement to which their particles have been subjected. As lava, obsidian, and pumice, are merely three states of the same volcanic product, so probably the granitic, porphyritic, and homogeneous rocks, generated by heat in ancient times, have derived their characteristic structures from the conditions of their solidification. ”

“ The older strata,—the strata which have been most deeply buried,—and especially those which have suffered most from folding and from fracture, show the largest amount of igneous intrusions. The molten rock which has been driven from the earth's interior to within the crust or to the surface during geologic time must be reckoned in millions of cubic miles. ”

“ Keeping in mind these general facts, but disregarding the crude notions which attribute to granite or trap rocks the elevations and fractures which have merely opened to us their subterranean repository, or given them channels to the surface, we shall be able to construct a table of the relative antiquity of igneous rocks, by comparing their distribution with the principal phenomena of convulsion in the crust of the earth. ”

“ The frequency of inclusions of the surrounding rocks, their rounded shapes indicating that they have been partly dissolved by the igneous magma, the intense alteration which they have undergone pointing to a state approaching actual fusion, the extensive changes induced in the rocks which adjoin the granite, the abundance of veins, and the unusual modifications of the granite which occur where it comes in contact with the adjacent strata, are adduced as evidence that there has been absorption and digestion of the country rock by the intrusive mass. ”

“ It has been the current opinion that the earth was once in a molten state, and thence cooled to a solid condition, and hence that all the primitive rocks were igneous. Even those who think that the earth may never have passed through a molten state agree that the oldest known rocks are either true igneous rocks, or rocks of very similar nature. ”

“ But by various forces they have been crumbled, some torn out, and in many places piled together. These layers, beginning at the bottom, are as follows; (1) igneous granite, unstratified; (2) limestone laid down from life in the ocean, metamorphosed by heat and all fossils thereby destroyed; (3) limestone highly crystallized, composed of fossil shells and very hard; (4) sandstone, made under the sea from previous rock powdered, having huge concretionary masses with a shell or a pebble as a nucleus around which the concretion has taken place ”

“ The igneous rocks are those which have solidified from a molten magma, like lavas, granites, etc. The sedimentary rocks are those which represent accumulations of fragments or grains, derived from various sources, usually the weathering of other rocks, and deposited by such agents as water, wind and organisms. Metamorphic rocks are those which were originally either igneous or sedimentary, but have been altered by the actions of heat, pressure and water, so that the primary character has been changed, often to such an extent as to be obscured. ”

“ Granite which we thus see irrupted through and into the stratified rocks, is, in fact, of no one particular or determinate age, but is the local result and evidence of many independent excitements or periods of critical action among the subterranean agencies of heat. We may, therefore, consistently admit granite, as well as other igneous rocks, to be of any, that is, of all ages; some of that which is visible in the crust of the globe may have been solidified from fusion before the production of any of the strata ”

“ An unconformity between two marine formations records the fact that between the periods when they were deposited in the sea the area emerged as land and suffered erosion (p. 227) . The attitude and structure of the strata tell also of the foldings and fractures, the deformation and the metamorphism, which they have suffered; and the igneous rocks associated with them as lava flows and igneous intrusions add other details to the story. Each formation is thus a separate local chapter in the geological history of the [292] earth, and its strata are its leaves. ”

“ The igneous rocks were first briefly compared by H. A. Brouwer. [3] He found no fewer than five parallels, namely, (1) the older granite, (2) the more recent granite, (3) alkaline rocks, (4) Jurassic volcanic rocks and intrusive dolerite, (5) kimberlite, alnöite, etc. ”

“ As is well known, however, igneous rocks are in some instances of value for the metallic mineral, gems, and ores associated with them, but in the great majority of instances at least, and as a rule, these minerals and ores are the result of subsequent changes and owe their origin mainly to deposition from heated, percolating water. ”

“ Igneous Rocks.—Rocks formed by the cooling and solidification of a hot liquid material, known as magma, that has originated at unknown depths within the earth. Those that have solidified beneath the surface are known as intrusive rocks, or if the cooling has taken place slowly at great depth, as plutonic rocks, e.g. granite. Those that have flowed out over the surface are known as effusive rocks, extrusive rocks, or lavas, e.g., basalt. Volcanic rocks include not only lavas, but bombs, pumice, tuff, volcanic ash, and other fragmental materials or ejecta thrown out from volcanoes. ”

“ No less important is the nature of the rocks involved. Where a granite intrudes into a succession of various types of sedimentary and igneous rocks the differences in their behaviour are often very marked. Sandstones alter less readily than shales or slates, and limestones, especially if they be marly or argillaceous, are often full of new minerals, when purer shales on each side of them are not visibly affected. ”

“ I have mentioned the occurrence of granite-veins conformable with the strata among which they lie. The claystone-porphyry of Bain, appears conformable as to individual beds, while in the mass it crosses the section of the country. I have never found igneous rocks in the positions of upheaving rocks. ”

“ The igneous rocks themselves furnish desirable building stones, such as granite, diorite, porphyry, diabase, etc. With the exception of granite and the nearly related diorite, these have not as yet been extensively utilized in North America. Certain of the igneous rocks have been altered to serpentine, which on account of its pleasing green colour and the ease with which it can be cut and 332 polished furnishes a stone valuable for interior uses. It is also employed, usually with a rough surface, in the construction of exterior walls of dwellings, gateways, etc. ”

“ Frequency of eruptions, and nature of subterranean igneous rocks.—When we speak of the igneous rocks of our own times, we mean that small portion which, in violent eruptions, is forced up by elastic fluids to the surface of the earth,—the sand, scoriæ, and lava, which cool in the open air. ”