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Show CHAPTER IV. THE CLASSIFICATION OF VOLCANIC EOCKS. Objects to be gained by a system of classification.â€"Artificial and natural systems.â€"The best system represents with accuracy the existing knowledge.â€"Progress is from the artificial to the natural classifications.â€"All are evanescent and temporary.â€"Classification of volcanic rocks chiefly with reference to physical properties.â€"Transitions to porphyritic rocks.â€"Correlations between physical properties.â€"Chemical composition.â€"Mineral ingredients.â€"Texture.â€"Density.â€"Fusibility.â€" Wholly crystalline and partly crystalline textures.â€"Texture as correlated to geological age of eruptions.â€"Not universally a true correlation.â€"Pre-Tertiary lavas common.â€"Von Cotta's view adopted.â€"View tested by comparison with facts.â€"Magmas of all ages the same.â€"Texture duo to conditions of solidification.â€"Porphyritic texture.â€"Difficulty of definition.â€"No strict demarka-tion between porphyries and lavas.â€"Crystalline rocks.â€"Significance of the wholly crystalline texture.â€"The two original groups.â€"Acid and basic rocks.â€"Subdivision of each.â€"Andesitc.â€" Rhyolite.-^The four major groups.â€"Conspectus of minerals characterizing the primary divisions.â€" Ehyolites.â€"Trachytes.â€"Andesites.â€"Basalts.â€"General system. The objects to be gained by a good system of classification I hold to be mainly two: first, accuracy of designation; and, second, convenience of treatment. In speaking of any natural object, it is desirable to indicate by a single word as much as possible concerning the attributes and relations of that object, and to avoid as far as possible all confusion with the attributes and relations of other objects. In order to secure this accuracy and convenience it is necessary that a classification should be so constructed as to express both the differences and community of attributes and relations. Where the differences of attributes between two or more objects are small and the community of relations is nearly complete, these objects are grouped together as to most of their features, and separated only by small distinctions, as varieties or species. Where these differences are very great, and the community very highly generalized, they are separated by much broader divisions, as in orders or classes. When a category of objects is once classified and familiarized to the mind, the mention of any one of them will convey not only an idea of the concrete object itself as an individual, but also 32 |