THE CLASSIFICATION OF THE IGNEOUS ROCKS 7, 
distinguished from the diorites by a gain of alkalis, lime and 
iron, and a loss of silica. 
The affimties of the syenites are seen to be entirely with the diorites 
and gabbros, with which they form a third progressive series which is 
continued imperfectly in the pyroxenites. In this series, characterized 
by generally decreasing silica and potash, the magnesia, lime, iron, 
and alumina increase, soda remaining practically constant throughout. 
The pyroxenites and peridotites, so poor in alkalis and alumina, 
show close affinity with each other and with the gabbros. 
A few petrographical curiosities are represented in Plate III 
— rocks so exceptional in their occurrence as to be almost or 
quite unique. The first of these is Rockallite from Rockall 
Bank in the northeastern Atlantic,* a rock of granitic tex- 
ture chemically closely related to the pantellerites of Firstner 
(see Plate VII); Urtite is a nearly pure nephelene rock from the 
Kola peninsula? in arctic Russia which forms the limiting mem- 
ber of the nephelene-syenite family. Yogoite from Montana; 
iseay asic wsyemite., Nhe “basic miephelene-syenite . from 
Beemerville, N. J.,* furnishes the most symmetrical of all the 
diagrams and gives indication of no near relationship to any 
other specific rock type though it is classed with the nephelene- 
syenites. The dunite from Elliott county, Kentucky 5 is so low 
in silica and so high in magnesia as to be very exceptional, 
though its diagram conforms to the general shape of the peri- 
dotite composite. Ijolite® and Missourite’, the two recently 
tJoHN W. JuDD: Notes on Rockall Island and Bank (Notice of Memoir) Geol. 
Mag., Dec., (4), VI, pp. 163-167, 1899. 
2 WILHELM RAmsAy: Urtit, ein basisches Endglied der Augitsyenit-Nephelin- 
syenit-Serie. Geol. Foren. Stockh. Foérh., XVIII, pp. 459-468, 1896. 
3 WEED and PirssoON : The Bearpaw Mountains of Montana, Am. Jour. Sci., (4), 
I, p. 357, 1896. 
4J. F. Kemp: A basic Nephelene-syenite from Beemerville, N. J., N. Y. Acad. of 
Sci., XI, p. 68, 1892. 
SJ. S. DILLER: The Peridotite of Elliott County, Ky. Bull. No. 38, U. S. Geol. 
Survey, pp. 1-29, 1887. 
6 WILHELM Ramsay : loc. cit. 
7 WEED and PIRSSON: Missourite, a New Leucite Rock from the Highwood 
Mountains of Montana. Am. Jour. of Sci., (4), Il, pp. 315-325, 1896. 
