GENETIC RELA TIONSHIPS AMONG IGNEO US ROCKS. 837 



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 pecu- 

 liarities 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 Brogger, 2 from the region of Chris- 

 tiania. They are characterized by a high percentage of sodium 

 and a consequent abundance of alkali minerals. Brogger 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. Brogger : Die Mineralien der Syenitpegmatitgange der Siidnorwegischen 

 augit- und nephelinsyenite. Zeitschr. fiir Kryst. u. Min., 8vo, Leipzig, 1890, Vol. 

 XVI., p. 83. 



3 H. Rosenbusch : Microskopische Physiographic der massigen 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. XIL, 8vo, pp. 89-214, PI. 2. Washington, 1892. 



