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GENETIC RELATIONSHIPS AMONG IGNEOUS ROCKS.
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whole groups or associations of rocks which distinguished them from groups in other regions. This was noticed by Judd in studying the volcanic rocks of Hungary and Bohemia, and was afterwards clearly expressed by him in defining petrographical provinces as districts "within which the rocks erupted during any particular geological period present certain well-marked peculiarities in mineralogical composition and microscopical structure, serving at once to distinguish them from the rocks belonging to the same general group, which were simultaneously erupted in other petrographical provinces."[1] A striking illustration of the individuality of a petrographical province is found in the unusual group of rocks described by Brögger,[2] from the region of Christiania. They are characterized by a high percentage of sodium and a consequent abundance of alkali minerals. Brögger calls attention to the remarkable fact that the greater part of the rocks in this district are absolutely peculiar to the locality, or nearly so, and have not yet been found in any other part of the world. The association of special kinds of rocks in different localities has also been pointed out by Rosenbusch,[3] and urged as evidence of a genetic relation between the rocks so grouped.

Certain chemical characteristics of special geographical groups of rocks become apparent when all of the chemical analyses are systematically compared and their variations plotted graphically, as has been done by the writer for the rocks of particular localities in the Yellowstone National Park, and for those of Vesuvius and vicinity, and of Pantellaria.[4] It is observed in these cases that the relations of the alkalies to one

  1. ↑ J. W. Judd: On the Gabbros, Dolerites and Basalts of Tertiary Age in Scotland and Ireland.Quart. Jour. Geol. Soc., Vol. 42, p. 54, 1886.
  2. ↑ W. C. Brögger: Die Mineralien der Syenitpegmatitgänge der Südnorwegischen augit- und nephelinsyenite.Zeitschr. für Kryst. u. Min., 8vo, Leipzig, 1890, Vol. XVI., p. 83.
  3. ↑ H. Rosenbusch: Microskopische Physiographie der unassigen Gesteine, 8vo, Stuttgart, 1886, pp. ix., 600, 628, 767, 795, 809, 810, 821.Also in Mineral. und petrogr. Mitth. XI., 1890, p. 445.
  4. ↑ J. P. Iddings: The Origin of Igneous Rocks.Phil. Soc. Washington, Bull. Vol. XII., 8vo, pp. 89-214, Pl. 2.Washington, 1892.
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