_Gabbro_, a typical basic rock, corresponds to basalt in chemical composition. It is a dark, heavy, coarsely crystalline aggregate of feldspar and _augite_ (a dark mineral allied to hornblende). It often contains _magnetite_ (the magnetic black oxide of iron) and _olivine_ (a greenish magnesian silicate). In the northern states all these types, and many others also of the vast number of varieties of intrusive rocks, can be found among the rocks of the drift brought from the areas of igneous rock in Canada and the states of our northern border. [Illustration: Fig. 249. Ground Plan of Dikes in Granite. (Scale 80 feet to the inch) What is the relative age of the dikes _aa_, _bb_, and _cc_?] [Illustration: Fig. 250. _A_ and _B_. Mountains of coarsely Crystalline Igneous _i_, surrounded by Sedimentary Strata _s_ and _s´_ Copy each diagram and complete it, so as to show whether the mass of igneous rock is a volcanic neck, a boss, or a laccolith] =Summary.= The records of geology prove that since the earliest of their annals tremendous forces have been active in the earth. In all the past, under pressures inconceivably great, molten rock has been driven upward into the rocks of the crust. It has squeezed into fissures forming dikes; it has burrowed among the strata as intrusive sheets; it has melted the rocks away or lifted the overlying strata, filling the chambers which it has made with intrusive masses. During all geological ages molten rock has found way to the surface, and volcanoes have darkened the sky with clouds of ashes and poured streams of glowing lava down their sides. The older strata,--the strata which have been most deeply buried,--and especially those which have suffered most from folding and from fracture, show the largest amount of igneous intrusions. The molten rock which has been driven from the earth's interior to within the crust or to the surface during geologic time must be reckoned in millions of cubic miles. [Illustration: Fig. 251. 1, limestone; 2, tuff; 3, 5, 7, shale with marine shells; 4, 6, lava, dotted portions scoriaceous. Give the history recorded in this section] [Illustration: Fig. 252. _a_, sedimentary strata with intrusive sheets; _b_, sedimentary strata; _c_, lava flow; _d_, dike. Give the succession of events recorded in this section] [Illustration: Fig. 253. Which of the lava sheets of this section are contemporaneous anti which intrusive,--_A_, whose upper surface is overlain with a conglomerate of rolled lava pebbles; _B_, the cracks and seams of whose upper surface are filled with the material of the overlying sandstone; _C_, which breaks across the strata in which it is imbedded; _D_, which includes fragments of both the underlying and overlying strata and penetrates their crevices and seams?] [Illustration: Fig. 254. Mato Tepee, Wyoming