42 RICHTHOFEN—NATURAL SYSTEM 
regard to them should be drawn with care. In the middle part of Germany, 
and in southern Tyrol, where they have been repeatedly studied, subjected to 
chemical analysis, and described in numberless treatises, quartzose porphyry is 
rather predominant. But porphyrite, melaphyr, and augitic porphyry, are, in the 
aggregate, little subordinate in bulk. The volcanic offers the complete reverse 
of the granitic era, respecting the proportionate quantity in which the different com- 
pounds have come to the surface. Andesite and basalt compose as large a proportion 
of the aggregate bulk of volcanic rocks, as granite and syenite do of those of the 
granitic era. 
Some minor differences in age may be noticed among the different orders com- 
posing the three classes of eruptive rocks. Extrusions of granite and syenite appear 
to have been almost the exclusive feature of the eruptive activity during granitic eras, 
and to have been succeeded only towards their close by the emission of diorite and 
diabase, or of other rocks of limited occurrence, such as gabbro and hypersthenite, 
Such at least has been observed to be the case in several countries, in regard to the 
chief outbreaks ; but if we enter into the details of the mode of succession of the 
rocks belonging to the different orders, we perceive that it has not been so definite 
as with volcanic rocks—granite and syenite bearing evidence, in many localities, of a 
more recent age than some neighboring masses of basic rocks. Yet, in keeping only 
the main features in view, we may easily see, that the general order of succession of 
granitic rocks has been conformable to a gradual decrease in silica. Syenite is usually 
more recent in origin than granite; and even among the different varieties of the lat- 
ter, true granite, containing the highest ratio of silica, has generally been anterior in 
age to G. Rose’s granitite. There may be some connection between these relations 
as they are exhibited in any single granitic district, and the fact that the granitic rocks 
of the Sierra Nevada, belonging altogether to a more recent era, contain no true granite— 
their chief bulk consisting of rocks intermediate in composition between granitite and 
syenite. The porphyritic era, in Germany and on the southern slope of the Alps, was 
inaugurated by eruptions of quartzose porphyry, and has terminated in the Alps by 
those of augitic porphyry. The intermediate epoch has been distinguished by rocks 
intermediate in composition. The mode of succession of the different orders is more 
distinct than with granitic, but less so than with volcanic rocks. Melaphyr and por- 
phyrite interchange frequently ; but, at many places, the former appears to have pre- 
ceded the latter in age, in a similar way as andesite preceded trachyte. They form’ 
together one epoch, which, in the commencement, was occasionally interrupted by 
an outbreak of quartzose porphyry.—The laws of the periodical succession of the five 
orders of volcanic rocks have been developed in the foregoing pages. Their epochs 
are much more distinctly separated than those of the ancient rocks, but the mode of 
their succession is more complicated. With granitic rocks, silica as a component part 
decreases with the age; with porphyritic rocks the same is true in a broad sense—the 
precedence in age of melaphyr and porphyrite forming the only conspicuous devia- 
tion ; while in reference to the voleanic era, no rule at all may be discovered at first 
sight. Considering, however, the predominant rocks of that era, which are propylite, 
(80) 
