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?

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.

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 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
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? Fig. 254. Mato Tepee, Wyoming This magnificent tower of igneous rock three hundred feet in height has been called by some a volcanic neck. Is the direction of the columns that which would obtain in the cylindrical pipe of a volcano? The tower is probably the remnant of a small laccolith, an outlying member of a group of laccoliths situated not far distant

The Interior Condition of the Earth and Causes of Vulcanism and Deformation