In popular usage, a dike is a wall or embankment constructed to control the threatening surge, tide, rise, or ﬂow of a sea or river. In geology, a dike is a mass of molten rock that has penetrated vertically through layers of older rock and solidiﬁed, forming an intrusion often harder than the surrounding rocks. (A similar horizontal intrusion forms a sill.) Dikes form either when volcanic magma rises through an existing fracture in the country rock, or when it generates a new crack by exploiting a weakness in existing rock. Dikes often appear as dark-colored vertical bands set in rock of a lighter color. Hundreds of dikes might intrude in the cone and inner core of a volcano, forming a dike swarm. The Independence Dike Swarm is one of the largest and most visually striking dike formations in North America, extending all the way from the White Mountains of the southern Sierra Nevada to the southern Mojave Desert in eastern California, offering some of the most beautiful vistas in the American landscape.