LACCOLITH, in geology, the name given by G. K. Gilbert to intrusive masses of igneous rock possessing a cake-like form (from Gr. Xlmos, cistern, Mos, stone), first described from the Henry Mts. of southern Utah. Their characteristic is that they have spread out along the bedding planes of the strata, but are not so broad and thin as the sheets or intrusive sills which, con sisting usually of basic rocks, have spread over immense dis tances without attaining any great thickness. Laccoliths cover a comparatively small area and have greater thickness. Typi cally they have a domed upper surface while their base is flat. In the Henry Mts. they are from 1 to 5 m. in diameter and range to 5,000 ft. in thickness. The cause of their peculiar shape appears to be the viscosity of the rock injected, which is usually of intermediate character and comparatively rich in alkalis, be longing to the trachytes and similar lithological types. These are much less fluid than the basalts, and the latter in conse quence spread out much more readily along the bedding planes, forming thin flat-topped sills. At each side the laccoliths thin out rapidly so that their upper surface slopes steeply to the margins. The strata above them which have been uplifted and bent are often cracked by extension, and as the igneous ma terials well into the fissures a large number of dikes is produced. At the base of the laccolith, on the other hand, the strata are flat and dikes are rare, though there may be a conduit up which the magma has flowed into it. The rocks around are often much affected by contact alteration, and great masses of them have sometimes sunk into the laccolith, where they may be partly melted and absorbed, with resulting changes in physical and chemical composition.
Gilbert found that these laccoliths were filled at depths of 7,000 to 1 o,000 ft. and did not reach the surface, giving rise to volcanoes. From the effects on the drainage of the country it seemed probable that above the laccoliths the strata swelled up in flattish eminences. Often they occur side by side in groups belonging to a single period, though all the members of each group are not strictly of the same age. One may be formed on the side of an earlier one, and compound laccoliths also occur. When exposed by erosion they give rise to hills, and their appearance varies somewhat with the stage of development.
In western South America similar laccoliths occur in consider able numbers and present some diversity of types. Occasionally they have one steep side while the other is gently inclined ; some are split into a number of sheets spreading outwards through the rocks around.
In Britain and elsewhere the term is used of a variety of in trusive masses not strictly identical in character with those of the Henry Mts. Some of these rest on a curved floor, like the gabbro masses of the Cuillin Hills in Skye ; others are injected along a flattish plane of unconformability where rock rests on the upturned and eroded edges of an older series, as in the felsite mass of the Black Hill in the Pentlands, which has followed the line between the Silurian and the Old Red Sandstone, forcing the rocks upwards without spreading laterally to any great extent.