Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 712 MINERAL STRUCTURE OF [Ch. xxxni. In the plutonic, as in the volcanic rocks, there is every gradation from a tortuous vein to the most regular form of a dike, such as inter- sect the tuffs and lavas of Vesuvius and Etna. Dikes of granite may be seen, among other places, on the southern flank of Mount Battock, one of the Grampians, the opposite wall sometimes preserving an exact parallelism for a considerable dist
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 712 MINERAL STRUCTURE OF [Ch. xxxni. In the plutonic, as in the volcanic rocks, there is every gradation from a tortuous vein to the most regular form of a dike, such as inter- sect the tuffs and lavas of Vesuvius and Etna. Dikes of granite may be seen, among other places, on the southern flank of Mount Battock, one of the Grampians, the opposite wall sometimes preserving an exact parallelism for a considerable distance. As a general rule, however, granite veins in all quarters of the globe are more sinuous in their course than those of trap. They pre- sent similar shapes at the most northern point of Scotland, and the southernmost extremity of Africa, as the foregoing drawings (figs. 741 and 742) will show. It is not uncommon for one set of granite veins to intersect another; and sometimes there are three sets, as in the environs of Heidelberg, where the granite on the banks of the river ISTecker is seen to consist of three varieties, differing in color, grain, and various peculiarities of mineral composition. One of these, which is evidently the second in age, is seen to cut through an older granite ; and another, still newer, traverses both the second and the first. In Shetland there are two kinds of granite. One of them, com- posed of hornblende, mica, felspar, and quartz, is of a dark color, and is seen underlying gneiss. The other is a red granite, which pene- trates the dark variety everywhere in veins.* The accompanying sketches will explain the manner in which gran- ite veins often ramify and cut each other (figs. 743 and 744). They Fig. 743. Granite veins traversing gneiss at Cape Wrath, in Scotland. (MacCulloch.) represent the manner in which the gneiss at Cape Wrath, in Suther- landshire, is intersected by veins. The light color strongly contrasted with that of the hornblen
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