OF VOLCANIC ROCKS. 35 



An instance, which is instructive on account of its simplicity, is furnished by 

 the island of St. Paul, in the Indian Ocean. F. von Hochstetter found its foundation 

 to consist of rhyolitic rocks. These are intersected by basaltic dykes. Rhyolite over- 

 lays the first basaltic formation, and is itself superposed, first by dolerite and then 

 again by basalt. These two rocks of the basaltic order constitute the main body of 

 Hie island, and encircle its crater. Similar relations, though on a much grander scale, 

 have been observed by the same eminent geologist on New Zealand. More frequently 

 than this order of succession between rhyolitic and basaltic lava, has been observed 

 the sequence of basalt to trachyte, with the omission of rhyolite, or immediately to 

 andesite, when both those rocks are absent. Vesuvius is built up of rocks of the 

 basaltic order, and still emits lava corresponding in mineral character to its predeces- 

 sors, while the rocks of its surroundings (Campi Phlegra-i), on the prior origin of 

 which geologists agree, are trachytic. The industrious explorer of Mount Etna, Sarto- 

 rius von Waltershausen, has described its foundation as being composed of white and 

 reddish colored trachytic rocks, which contain hornblende as a characteristic ingred- 

 ient, while among those rocks which build up the summit, as also in all modern lava of 

 the volcano, no hornblende but, in its place, augite is visible. This mineral and labra- 

 dor compose the recent lava, which belongs to the basaltic order. The much more 

 extensive recurrence of a similar order of succession in the Eifel and in Auvergne, 

 is too well known from the accurate descriptions of the geology of those regions, to 

 require to be here more fully mentioned. It contributes especially to confirm our 

 proposition, that the volcanoes of the different orders, as regards their origin, have 

 been nearly contemporaneous with the correlated massive eruptions. The classical 

 descriptions of the Eifel, by Mr. von Dechen, give conclusive evidence thereof. 



Among those volcanoes the lava of which has never undergone a material 

 change and is, at the same time, similar in nature over the*area of larger volcanic 

 districts, may be mentioned, besides numerous basaltic volcanoes, those of the trachytic 

 order in Central, and those of the andesitic order in South America, as far as may be 

 seen from the descriptions given of them. 



We might greatly enlarge this enumeration of observations confirming our 

 propositions ; but, as by most authors only a " trachytic" or a " basaltic" character 

 of lava have been mentioned in a general way, they would only furnish evidence in 

 favor of the general tenor of the law, but would fail to give it in regard to any of its 

 details. 



Relation of Volcanic Rocks to Ancient Eruptive Rocks. 



All rocks which, bearing evidence of an intrusive or eruptive origin, preceded 

 in age the Tertiary period, may, by principles similar to those which we applied in 

 tracing the natural system of volcanic rocks, be divided into two great classes, for 

 which we may use the terms "granitic rocks" and "porphyritic rocks," derived from 

 the mode of texture predominating in either class. Granitic rocks are, besides, 

 geologically associated with granite, which is their principal type, while quartzose 

 porphyry occupies a similar position among porphyritic rocks. The annexed table 



(73) 



