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Show 92 GEOLOGY OF THE HIGH PLATEAUS. therefore, form very slowly, and time becomes an important element in determining the whole amount of crystallization. It is easy to see that an eruptive lava, rapidly cooling under the sky, may remain but a short time at the temperatures at which crystals can form. On the other hand, an injected or plutonic mass may long retain its high temperature. In the former case the rock finally becomes half-crystalline, in the latter case wholly crystalline. That this is the explanation of the textural differentiation of the plutonic and erupted rocks seems very probable, and thus texture becomes associated with the genesis of the rock and the causes which have made it what it is. There is a very respectable school of German lithologists who make the geological age of igneous rocks a primary criterion of classification. They place all igneous rocks, whose intrusion or eruption occurred prior to Tertiary time, among the granitoid or porphyroid classes, and all Tertiary or Quaternary eruptives among the true volcanics. For example, all augitic plagioclase rocks of Pre-Tertiary origin are regarded as diabases, mela-phyres, or augitic porphyries, &c, while all of Post-Cretaceous origin are regarded as basalts, " trachydolerites," &c. Such a classification most assuredly could be defended only upon the assumption or ascertained fact that certain characters are found in the more ancient eruptives which are wanting in the more recent ones and vice versa. Is this assumption universally true ? I hold that it is not. That in a great majority of cases the Pre-Tertiary igneous, as we now see them, are granitoid or porphyroid, while those of later epochs are volcanic, thus presenting textural differences, is undeniable. But exceptions exist, and they are highly important ones. It is possible, not to say probable, that many more exceptions might be looked for than can at present be specifically named if there were not a certain looseness in the use of names, by which rocks of the volcanic texture are classified with the granitic groups. This is especially observable in the augitic divisions. The augitic rocks of the Palaeozoic system, notably those of Carboniferous age, are frequently classed as diabase, when more properly they might be in many instances placed among the dolerites or basalts. Indeed, some intelligent observers, who are not committed in any way to the foregoing generalization, do not scruple to call the intruded and |