42 RICIITHOFEN 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 volcanic era, no rule at all may be discovered at first 

 sight. Considering, however, the predominant rocks of that era, which are propylite, 



(80) 



