6o JOURNAL OF GEOLOGY—SUPPLEMENT 



essexite. Such a tendency in naming rocks is to be guarded against, 

 for it covers up facts — ^in this case the fact of frequent association 

 of normal gabbroic rocks with alkaline types. 



LEUCITE -BEARING ROCKS 



Rocks bearing leucite are of rare occurrence and their relations 

 are, perhaps, not well enough understood to make clear the manner 

 of their origin. Leucite is sometimes one of the minerals pre- 

 cipitated from a nephelite syenite magma derived, presumably, in 

 the manner already outlined. The relations of some leucite-bearing 

 rocks suggest, however, that differentiation in the magma from 

 which they were formed proceeded along the lines outhned in the 

 discussion of the formation of biotite and that the magma was then 

 intruded to a high level where water could escape more or less 

 freely. The lowering of the pressure of water-vapor lessens the 

 possibility of the formation of those hydrous molecules which go to 

 form mica, and a part of the potash must then be precipitated in 

 some form other than mica, viz., as leucite."^ Highwood Peak 

 stock, for example, sends out mica-rich dykes, and these, where 

 they cut loose breccias which would facilitate the escape of water, 

 often pass in a single dyke into a leucite-bearing rock, so-called 

 leucite basalt.^ Iddings has called attention to the fact that some 

 minettes and some leucite basalts have nearly identical composition. 

 Shonkinite, a type comparatively rich in biotite, has no mineralogi- 

 cal equivalent among the effusive rocks, but is represented by its 

 chemical equivalent, which is a leucite rock, owing presumably to 

 the loss of water on extrusion. It seems possible that the gradual 

 escape of water from the outer border of the Shonkin Sag laccolith 

 has given rise to the 5-15-foot border of leucite-basalt which is 

 nearly identical in composition with the shonkinite immediately 

 adjacent to it.^ If the partial escape of water takes place from a 

 moderately deep-seated body, even though not especially rich in the 

 molecules which go to form mica, it seems possible that further 



^ Cf. H. S. Washington, "The Formation of Leucite in Igneous Rocks," Jour. Geol., 

 XV (1907), 377-79- 



^ L. V. Pirsson, U.S. Geol. Survey, Bull. 237, 1905, p. 22. 

 3 Pirrson, Op. cit., p. 47. 



