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THE POPULAR SCIENCE MONTHLY.
tives 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
Fig. 11.—Plan of the Dissected Volcano of Mull, in the Inner Hebrides.
| a Rocks, on which the volcano has been built up. | b Great intrusive masses of acid and intermediate rocks. | c Lava-currents of basalt which have flowed from d. | d Intrusive maasses of gabbros and dolerite. | r Lava-currents which have flowed from b. | f Volcanic tuffs and agglomerates. | |||||
| Fig. 12.—Section of the Volcano along the line A B. | ||||||||||
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 pene-
