In the formation of volcanic mountains there is an extrusion of molten and fragmentary material accompanied by an escape of great volumes of steam at the surface. Closely related to this phase of vulcanism is the injection of molten material into the earth's crust from below, so as to force its way between stratified beds and produce intruded sheets. In this latter process the sheets of injected material may be thin in comparison to their lateral extent or thick lens-shaped masses. In the production of either of these forms of intrusion the cover above the injected material is lifted and a change is made in the topography of the surface. The intrusions, which are thick in comparison to their lateral extent, are known as laccoliths. At times intrusions of this nature are of such thickness that they produce true mountain forms. If unmodified, these elevations would be domes, but when their surfaces are broken by erosion and their dissection and removal progresses they frequently assume rugged, serrate forms.
The type of laccolithic mountains made known some years since by the studies of G. K. Gilbert is furnished by the Henry Mountains, in southern Utah. More recently it has been found that this interesting phase of mountain building is illustrated by many other examples in the Rocky Mountain region and elsewhere.
The rocks exposed at the surface in the southern Rockies, as in the northern division of that great chain, embrace almost every variety which enters into the composition of the earth's crust. 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.
The High Plateaus.—To the west and south of the Park Mountains, and situated in the western portions of Colorado and New Mexico, eastern and southern Utah, and northern Arizona, there is an extensive region having a general surface level of 6,000 to 7,000 feet above the sea, known as the High plateaus. This region has suffered great erosion and is deeply trenched by stream-carved cañons. Although not mountainous in the ordinary acceptance of the term, its surface is rugged and difficult to traverse, particularly on account of the deep cañons that intersect it in every direction.
The High plateaus are a part of the Rocky Mountain region and bear a somewhat similar relation to the Park Mountains that the Alleghany plateau does to the Appalachian Mountains. Streams flowing westward from the Park Mountains and from the southwestern portion of the Stony Mountains unite to form the Colorado River—the one great drainage channel of the region. The importance of this remarkable river in the history of the land has led to the adoption of the name Colorado plateau by Powell and others for the region under consideration. As with so many of the grander geographical units of the continent, the precise limits of the one here considered are difficult to define; but in spite of this uncertainty as to meets and bounds, the now classical writings of Newberry, Powell, and Dutton especially have shown that a strange and wonderful land exists in the southwest part of the United States, which is of unusual interest to geologists and geographers.
The High plateaus are underlaid by nearly horizontal rocks. The larger elements in the structure are great blocks of the earth's crust measuring some 60 to 100 miles on their various borders, which are bounded by breaks (faults), or by what are termed monoclinal folds or a change from one plateau to another by a single bend in the strata. The rocks in each of the separate plateaus are usually gently tilted. Their eroded edges stand as lines of massive, gorgeously coloured, and frequently fantastically sculptured cliffs. These cliffs, when seen from below, appear as rugged mountain ranges, but to an observer standing on
their deeply sculptured crests are easily recognised as the upturned edges of large gently tilted blocks of the earth's crust.
The basement rocks beneath the High plateaus are very ancient granites, schists, etc., which formed a land surface and were greatly eroded before the first of the superimposed stratified beds were deposited upon them. The first of the sheets of sediment laid down by the primeval ocean belong to the oldest rocks containing records of life that have as yet been recognised—the Algonkian (pre-Cambrian) terranes of modern geology. Above these come other deposits of sandstone, shales, limestone, etc., representing a wide range of geological history, and including as the upper member of the series the sediments of large Tertiary lakes. This vast succession of stratified rocks, some 13,000 feet in thickness, has been upraised in a broad way, without the crumpling and folding, but broken, as stated above, into great blocks which are now variously inclined, but still preserve a plateau-like character.
Besides the movements in the earth's crust which raised the plateaus and caused fractures and simple or monoclinal folds in the rocks of which they are composed, there occurred volcanic eruptions which produced numerous cinder cones, extensive lava-flows, and widely spread sheets of comminuted material known as lapilli, dust, and so-called ashes.