A laccolith (or laccolite) is a large, lenticular mass of igneous rock, filling a chamber which it has made for itself by lifting the overlying strata into a dome-like shape; the magma was supplied from below through a relatively small pipe or fissure. The rock of which laccoliths are made is nearly always of the highly siliceous and less fusible kinds, so that it can more easily lift the strata than force its way between them. Intrusive sheets are, it is true, often given off from a laccolith, but these are of quite subordinate importance, while dykes and irregular protrusions, called apophyses, extend into the fissures of the surrounding and overlying strata. Subsequent erosion may remove the dome of strata and cut deeply into the igneous mass beneath, leaving rugged mountains, the height of which depends upon the amount of original uplift and the subsequent denudation. Laccoliths in various stages of denudation occur in different parts of the West. Fig. 219 shows Little Sun-Dance Hill in South Dakota, a small dome from which the overarching strata have not been removed and the igneous core has nowhere been exposed, yet there can be little doubt of its presence.

Bear Butte (Fig. 220) represents a second stage of denudation; the strata have been removed, except those upturned around the foot of the butte, and the igneous core, exposed* yet but little eroded. In the same region is Mato Tepee (also called the Devil's Tower), a magnificent shaft of columnar phonolite, which rises 700 feet above a platform of horizontal strata. This tower is the remnant of a laccolith from which the covering strata, and probably much of the igneous core, have been eroded away. In southern Utah the Henry Mountains are a group of laccoliths from which several thousand feet of overlying strata have been removed and the cores deeply dissected. In the Elk Mountains of Colorado are some enormous laccolithic masses.

Diagrammatic vertical section of a laccolith (Gilbert). The full black indicates igneous rock.

Fig. 216. - Diagrammatic vertical section of a laccolith (Gilbert). The full black indicates igneous rock.

Eroded laccolith, with many sills and apophyses; Colorado. (Holmes).

Fig. 217. - Eroded laccolith, with many sills and apophyses; Colorado. (Holmes).

Vertical section through laccolith shown in Fig. 217 before denudation.

Fig. 218. - Vertical section through laccolith shown in Fig. 217 before denudation. aa, present surface: full black, remaining parts of intrusive body; vertical lines, portion of laccolith removed by denudation. (Holmes).

Little Sun Dance Hill, South Dakota. (U. S. G. S).

Fig. 219. - Little Sun-Dance Hill, South Dakota. (U. S. G. S).

Bear Butte, South Dakota. (U. S. G. S).

Fig. 220. - Bear Butte, South Dakota. (U. S. G. S).

Chonoliths

Sometimes the shape of an intruded igneous body is so irregular and its relations to the country rock are so complex that it cannot be referred to any of the preceding categories. For such irregular masses Daly has proposed the term chonolith, which he defines as follows: "an igneous body (a) injected into dislocated rock of any kind, stratified or not; (b) of shape and relations irregular in the sense that they are not those of a true dyke, vein, sheet, laccolith, bysmalith or neck; and (c) composed of magma either passively squeezed into a subterranean orogenic chamber, or actively forcing apart the country rocks." Chonoliths are probably much more numerous than true laccoliths.