A marked variation in the process just outlined occurs when, as the controlling condition, the intruded magma is highly viscous instead of highly fluid, and the friction of contact and of flow is greatly increased. Under such circumstances the intruded magma expands less widely than is the case when an intruded sheet is formed, and a thick intrusion results, which lifts a small cover perhaps to a great height. Intrusions of this nature are sometimes expanded in their upper portions into a more or less mushroom shape, and from their fancied resemblance to cisterns of once molten rock within older terranes have been termed _laccoliths_. The typical examples are furnished by the Henry Mountains in southern Utah, described by G. K. Gilbert. Other similar intrusions in Colorado have been studied by Whitman Cross, and yet other examples have been discovered in various parts of the Pacific mountains. In the case of certain of the laccoliths in the Henry Mountains, now laid bare by erosion, the cistern-like mass of intruded material is 12,000 feet or more in diameter, some 5,000 feet thick in the central part, and lifted a cover of stratified rocks fully 7,000 feet thick. Where a dike ends above in older rocks, and particularly in horizontally stratified sedimentary beds, in a pipe-like form, similar to the conduit of a volcano, but without reaching the surface, the unexpanded or but slightly enlarged summit portion lifts a comparatively small cover into a dome, and what has been termed a plutonic plug results. All the various phases of intrusions thus far referred to, it will be readily seen, are variations of one process. The wide range in the results produced are dependent on local conditions, either in respect to terranes invaded, as, for example, whether or not they are undisturbed sedimentary beds, and on the physical condition of the intruded material, in reference especially to its degree of viscosity. There is an intimate and even a genetic connection between intrusions on the one hand and volcanic and fissure eruptions on the other. If fissures lead from portions of the earth's crust sufficiently deep to permit the rocks to become plastic or fused on account of the relief of pressure due to the opening of the fissure, the magma may be forced to the surface, becoming more and more plastic or more perfectly fluid as the weight upon it decreased, and volcanic phenomena result; or if the fissure fails to reach the surface intrusions of various forms may be produced. The simplest form of intrusion, the dike, results under whatever condition the summit portion of the magma comes to rest. A magma forced upward in fissures in the earth's crust may meet moist rocks or even reservoirs of water, and in such instances steam or gases are produced and a new force is added, which may produce explosions.