. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 600 PORPHYRY. [Ch. xxvm. trachyte, which has 66 per cent, of silica, has a sp. gr. of only 2*68; trachytic porphyry, containing 69 per cent, of silica, a sp. gr. of only 2'58. If we then take a rock of intermediate composition, such as that prevailing in the Peak of Teneriffe, which Abich calls Trachyte- dolerite, its proportion of silica being intermediate, or 58 per cent., it weighs 2-78, or more than trachyte, and less than basalt.* The basalts are generally dark
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 600 PORPHYRY. [Ch. xxvm. trachyte, which has 66 per cent, of silica, has a sp. gr. of only 2*68; trachytic porphyry, containing 69 per cent, of silica, a sp. gr. of only 2'58. If we then take a rock of intermediate composition, such as that prevailing in the Peak of Teneriffe, which Abich calls Trachyte- dolerite, its proportion of silica being intermediate, or 58 per cent., it weighs 2-78, or more than trachyte, and less than basalt.* The basalts are generally dark in color, sometimes almost black, whereas the tra- chytes are gray, and even occasionally white. As compared with the granitic rocks, basalts and trachytes contain both of them more soda in their composition, the potash-felspars being generally abundant in the granites. The volcanic rocks moreover, whether basaltic or tra- chytic, contain less silica than the granites, in which last the excess of silica has gone to form quartz. This mineral, so conspicuous in gran- ite, is usually wanting in the volcanic formations, and never predomi- nates in them. The fusibility of the igneous rocks generally exceeds that of other rocks, for the alkaline matter and lime which commonly abound in their composition serve as a flux to the large quantity of silica, which would be otherwise so refractory an ingredient. We may now pass to the consideration of those igneous rocks, the characters of which are founded on their form rather than their com- position. Porphyry is one of this class, and very characteristic of the volcanic formations. When distinct crystals of one or more minerals are scat- tered through an earthy or compact base, the rock is termed a por- phyry (see fig. 675). Thus trachyte is porphyritic; for in it, as in Fig. 675. Fig. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearanc
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