successive accretions until at length its cooled mass, heavier and stronger than the surrounding rocks, proves a sufficient obstacle to intrusion. The next irruption then avoids it, opens a new conduit, and builds a new laccolite at its side. By successive shiftings of the conduit a group of laccolites is formed, just as by the shifting of vents eruptive cones are grouped. Each laccolite is a subterranean volcano. [Illustration: FIG. 53.—Diagram to illustrate the relation of Dikes and Sheets to the Strains which are developed in the uplifting of laccolitic arches. ] The strata above the laccolite are bent instead of broken, because their material is subjected to so great a pressure by superincumbent strata that it cannot hold an open fissure and is _quasi-plastic_. But although quasi-plastic it is none the less solid, and can be cracked open if the gap is instantaneously filled, the cracking and the filling being one event. This happens in the immediate walls of the laccolite, and they are injected by dikes and sheets of the lava. The directions of the cracks are normal to the directions of the extensive strains (strains tending to extend) where they occur. From the top of the laccolite dikes run upward into the roof, marking horizontal strains (_a a_). From the sides smaller vertical dikes run outward, marking horizontal, tangential strains. And parallel to the sides near the base of the laccolite, are numerous sheets, marking strains directed outward and upward (_c c_). These last especially serve to show that the rigidity of the strata is not abolished, although it is overpowered, by the pressure which warps them. Here we are brought face to face with a great fact of dynamic geology which though well known is too often ignored. The solid crust of the earth, and the solid earth if it be solid, are as plastic _in great masses_ as wax is in small. Solidity is not absolute but relative. It is only a low grade of plasticity. The rigidity or strength of a body is measured by the square of its linear dimensions, while its weight is measured by the cube. Hence with increase in magnitude, the weight increases more rapidly than the strength; and no very large body is strong enough to withstand the pressure of its own weight. However solid it may be, it must succumb and be flattened. When we speak of rock masses which are measured by feet, we may regard them as solid; but when we consider masses which are measured by miles, we should regard them as plastic. The same principle is illustrated by the limital area of laccolites. A small laccolite cannot lift its small cover, but a large laccolite can lift its correspondingly large cover. The strength or rigidity which resists deformation is overcome by magnitude.