Cr 
OF VOLCANIC ROCKS. 3 
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. Khyolite 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 
the 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 Phlegrai), 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 Hifel, by Mr. von Dechen, give conclusive evidence thereof. 
Among those voleanoes 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 voleanoes, 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 Voucantc Rocks to AnciENT Hruptive 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 roeks,” 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) 
