408 II. S. WASHINGTON — IGNEOUS COMPLEX OF MAGNET COVE 



Other instances. — Although this is contrary to the general rule, yet it is 

 by no means an isolated exception. Brogger* furnishes an example, 

 with five analyses, in the laccolithic mass of Ramnas. Here the center 

 is a medium grained akerite, with Si0 2 — 58.48, CaO = 5.02, and alka- 

 lies = 8.58. This gradually becomes finer grained and more acid as the 

 border is approached, till at the contact at Gislerud the rock is a fine 

 grained quartz-porphyry, with Si0 2 = 71.49, CaO — 0.30, and alkalies = 

 10.18. The ratio of Na 2 : K. 2 also varies continuously, as in Essex 

 county, Massachusetts, Magnet Cove, and elsewhere. 



The diorite stock of the Castle mountains, f Montana, may also be 

 mentioned. This is basic at the center, growing less so toward the 

 periphery, where it becomes a very acid quartz-porphyrite. 



Another instance is the laccolith of Umptek, Kola, in Finland, described 

 by Ramsay and Hackmann. J This is composed mainly of a peculiar 

 foyaite which has been called chibinite,§ showing a trachytoid structure, 

 abundant segirine, arfvedsonite, eudialyte, and other rare minerals, and 

 which is very high in Na 2 0.|| The mass is cut b}' sheets of finer grained 

 chibinite, theralite, and ijolite, suggesting the idea that they are possibly 

 intrusions of the still fluid basic interior through the already cooled 

 outer mass of chibinite. Lastly and most important, there occurs at the 

 borders of the mass a more acid, nepheline-poor to nepheline-freesyenite,^[ 

 which Ramsay calls umptekite and which is allied to the pulaskite of 

 Fourche mountain, Arkansas. This umptekite, it may be noted, is 

 classed by Ramsay ** under the endomorp/iic modifications of the main 

 chibinite. 



At this center, then, we apparently have a state of affairs closely re- 

 sembling those of Magnet Cove, both as regards the general chemical 

 characters of the rocks, their occurrence as a laccolith or originally 

 homogeneous mass of magma, and their order of arrangement. 



CA USE OF DIFFERENTIA TION 



General discussion of causes. — The exceptional character of these oc- 

 currences would seem to imply that some, at least, of the principles 

 usually invoked to explain differentiation can not apply here. I say 

 ''seem " since it must not be forgotten that it is possible, indeed prob- 

 able, that several processes may be involved, either simultaneously or 



* Brogger : Zcit. Kryst., vol. xvi, 1890, p. 45. 



f Weed and Pirsson : Bull. no. 139, U. S. Geological Survey, 1896, pp. 134 and 140. 



I Ramsay and Hackmann : Fennia. vol. 11, no. 2, Helsingfors, 1894. 



§ Brogger : Erupt, gest. d. Krist. geb. iii, 1898, p. 29. 



|| See analyses later on page 412. 



TI Ramsay and Hackmann : Op. cit., pp. 204 and 214, and figs. 13 and 14, p. 75. 



** Ramsay and Hackmann : Op. cit., p. 197. 



