Fig. 380.—Columnar structure, obsidian cliff, Yellowstone Park. (Iddings, U. S. Geol. Surv.)
When lava is forced into crevices or rises to the surface through fissures, and the residual portion solidifies in them, it gives rise to dikes, as illustrated in Figs. [2] and [417] (not a true dike). Dikes are sometimes affected by columnar structure. In this case, as in all others, the columns are likely to be at right angles to the cooling surface. Lava solidifying in the passageway leading from the interior of a volcano gives rise to a neck or plug. If the lava is forced between beds of rock in the form of a sheet, and solidifies there, it is called a sill. If, after rising to a certain point in the strata, the lava arches the beds above into a dome, and forms a great lens-like or cistern-like mass, it constitutes a laccolith ([Fig. 334]). If an intrusion of the laccolithic type faults the overlying beds instead of arching them, and especially if the vertical dimension of the intruded mass be great in comparison with its lateral dimensions, its shape is more like that of a plug or core. Such an intruded core is a bysmalith[213] ([Fig. 124]). Between the bysmalith and the laccolith there are various gradations, just as between the laccolith and the sill. When lava forces aside the rocks at considerable depths or absorbs them by solution or by “stoping,” and then solidifies in great masses of irregular or undetermined forms, these masses are called batholiths.
Fig. 381.
Fig. 382.
Fig. 383.
Fig. 381.—Sections of columns from Giant’s Causeway, coast of Ireland.
Fig. 382.—Ball-and-socket joints in columns of basalt. (Scrope.)
Fig. 383.—Diagram to illustrate the first stages in the formation of hexagonal columns by contraction.
