OF VOLCANIC ROCKS. 7 
sumed as the point of issue. Taking crystalline texture, lack of stratification as well 
as of foliation, and the fact of their being made up of silicates to be the characteristic 
features of eruptive rocks in general, the most obvious external differences among 
them are caused by the variations of texture and color. During the early stages of 
petrographical science, rocks were therefore classified on these principles. The terms 
trap, porphyry, pearlstone, obsidian, lava, amygdaloid, wacke, as well as green- 
stone, black porphyry, green porphyry, and others, are the remnants, in our present 
nomenclature, of that epoch when the more minute differences of rocks arising from 
their mineral composition were but imperfectly investigated. This principle was 
necessarily next in order for serving as point of issue for classification, since, as far as 
regards external characters, it is only second to the former in value. The emer- 
gence of petrology from a chaotic state, by the scientific application of this prin- 
ciple, dates from the investigation of Gustav Rose on the feldspathic minerals entering 
into the composition of rocks, and it has since been more generally applied for 
establishing subdivisions than any other. The presence or absence of quartz, the 
predominance among the feldspathic minerals of orthoclase, oligoclase, or labrador, 
the presence of augite or hornblende, are the usual points of issue, even in the most 
recent attempts at classification. The high value of mineralogy as a basis of classi- 
fication cannot be denied. But its exclusive application has caused the combi- 
nation into certain groups, of such rocks as from a geological point of view are 
widely separated, while it has given rise to distinctions in cases where the results of 
geological observation would demand close connection, as we shall have occasion to 
illustrate in the following pages, with reference to those volcanic rocks which are com- 
posed of hornblende and oligoclase. Gradually, those differences based on chemical 
composition, not capable of being detected by the eye, and the knowledge of which 
could only be obtained after chemistry had made the necessary advauces, have become 
an object of scientific research. But this principle has not yet been used to any great 
extent for classification. It can easily be demonstrated that, when exclusively ap- 
plied, it leads to a systematical arrangement of rocks which is in even greater contra- 
diction with the natural mode of occurrence than when the same is based upon 
mineral composition alone, notwithstanding the fact that its great value has been con- 
clusively demonstrated, especially by the important results which Bunsen obtained 
from the chemical analysis of rocks, and which mark an era in petrology. To com- 
bine granite, quartzose porphyry, and rhyolite into one class, because they resemble 
each other in their chemical composition, and to place them at the head of the list 
because containing the highest amount of silica observed among eruptive rocks, would 
be to take no regard whatever of geological facts. Rhyolite is, mineralogically and 
geologically, far nearer related to trachyte than to either granite or quartzose por- 
phyry ; and these two are quite distinct from each other, while granite is closely allied, 
by gradual passage, to syenite, and quartzose porphyry to porphyrite. 
It is by slow degrees only that we can hope to reach a more scientific, that is, 
a more natural system in this, the most intricate branch of descriptive natural sciences. 
The natural differs from the artificial system in this, that it starts from the application 
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