Page:Popular Science Monthly Volume 20.djvu/372

358 of the first extruded lavas—are penetrated by intrusions of gabbro, the granitic form of the later ejected lavas. From these great intrusive masses of highly crystalline rocks proceed in every direction





spurs or dikes, which are evidently the radiating fissures formed during the outwelling of igneous materials from below, injected by fluid substances. Besides the vertical or oblique dikes we also find horizontal sheets, which, passing from the central masses, have