Where molten materials have been forced into cracks or fissures in the crust of the earth and there congealed, we have a very common mode of occurrence called "dikes" (Plate 9). In many regions often one set of dikes was formed, after which one or more succeeding injections from the same or different deep-seated bodies of molten rock took place, and some of the later dikes were forced to cut across earlier ones. Dikes of all lengths up to at least thirty miles, and of all widths up to many hundreds of feet, are known, but they are generally less than a mile long and not more than a few feet or rods wide. They have been intruded into all kinds of rock formations--igneous, sedimentary, and metamorphic. Dikes are common in many parts of the world and they often excite the interest of lay-men. They are wonderfully displayed along the southern coast of Maine. Plate 9 shows small dikes where the molten material was forced from a larger mass into a body of older dark rock. The Palisades of the Hudson River, just north of New York City, consists of a layer of igneous rock several hundred feet thick which, in the molten condition, was forced nearly horizontally between layers of sandstone millions of years ago, that is in the early Mesozoic era. The palisade or columnar structure was caused by cracking of the rock during the cooling and contraction. This is the explanation of most columnar structures of igneous rocks, exceptionally fine exhibitions being at the Giant's Causeway in Ireland, and Devil's Tower, Wyoming (Plate 10). A type of occurrence not so common, but of special interest, is where a body of molten rock rising in nearly horizontal strata becomes cooler and therefore stiffer or more viscous and, losing its power to penetrate, forces its way between the layers causing the strata to be arched or domed over it. Sufficient removal of overlying material by erosion has revealed many fine examples of this type of occurrence. Another type of interest is the volcanic neck, which is the core or plug filling the feeding channel of a volcano. In certain regions, like parts of Arizona and New Mexico, extinct volcanic mountains may be all cut away by erosion, except the central cores or necks which, both because they are more resistant and are last to be reached by erosion, stand out conspicuously as great towers on the landscape (Plate 9). Most important of all from the quantitative standpoint, however, are the great bodies of igneous rocks, ranging up to many miles across, which, in a molten condition, were forced irregularly into the earth's crust from unknown depths.