Sinopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1868 Excerpt: ...to the distinction of eruptive rocks. Observations in regard to them are abundant in some parts of Europe, fortunately in such countries as are especially capable of giving a clue to their general knowledge. Distinct conclusions may bo arrived at in regard to portions of that continent, but little scope is afforded for giving them latitude by comparison with the relations presented in other countries. We have, therefore, to keep well separate in general petrology, those positive conclusions which arc founded upon sufficient observations, and are applicable to the relations in those countries where the latter were made, and the realm of theories, which are arrived at, partly by the generalization of those conclusions, and partly by making deductions from hypothetical suppositions; because, the premises being founded on local occurrences, and their general validity not being proved, the theories must necessarily have a great deal of uncertainty, which will only gradually be dispelled by the advancing knowledge of the geology of the globe. The correlation of age and texture, as resulting from observations made in Europe, will occupy us first. Granitic rocks are widely distributed on that continent. Their great eruptive masses are of Azoic and Palaeozoic age. The rocks have almost throughout granitic texture, though the distinguishing features of porphyritic rocks are proper to some subordinate varieties of diorite and diabase. The eruptive activity exhibited in the granitic era gradually relaxed. It appears to have been insignificant, though it was by no means extinct, in the latter part of the Devonian and the first part of the Carboniferous periods. But it recommenced about the time of the deposition of the coal measures, thence increased in intensity, and w...
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Reseña del editor
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1868 Excerpt: ...to the distinction of eruptive rocks. Observations in regard to them are abundant in some parts of Europe, fortunately in such countries as are especially capable of giving a clue to their general knowledge. Distinct conclusions may bo arrived at in regard to portions of that continent, but little scope is afforded for giving them latitude by comparison with the relations presented in other countries. We have, therefore, to keep well separate in general petrology, those positive conclusions which arc founded upon sufficient observations, and are applicable to the relations in those countries where the latter were made, and the realm of theories, which are arrived at, partly by the generalization of those conclusions, and partly by making deductions from hypothetical suppositions; because, the premises being founded on local occurrences, and their general validity not being proved, the theories must necessarily have a great deal of uncertainty, which will only gradually be dispelled by the advancing knowledge of the geology of the globe. The correlation of age and texture, as resulting from observations made in Europe, will occupy us first. Granitic rocks are widely distributed on that continent. Their great eruptive masses are of Azoic and Palaeozoic age. The rocks have almost throughout granitic texture, though the distinguishing features of porphyritic rocks are proper to some subordinate varieties of diorite and diabase. The eruptive activity exhibited in the granitic era gradually relaxed. It appears to have been insignificant, though it was by no means extinct, in the latter part of the Devonian and the first part of the Carboniferous periods. But it recommenced about the time of the deposition of the coal measures, thence increased in intensity, and w...
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