OF VOLCANIC ROCKS. I 



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 iu 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 G-ustav 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 advances, have become 

 an object of scientific research. But this principle has pot 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 



