54 JOURNAL OF GEOLOGY— SUPPLEMENT 



toward a more basic nature of small bodies as compared with the 

 more acid nature (granodiorite and granite) of the larger bodies of 

 the main range. 



The same may be said of the gabbro-quartz monzonite series of 

 the Boulder batholith and its satellitic stocks in Montana. Again, 

 in the Sierra Nevada, as represented in the Pyramid Peak area 

 described by Lindgren,^ a series of the same type is shown. 



The relations of the various types of the great igneous complexes 

 of the Coast Range and of the Sierra Nevada seem to the writer 

 strongly to favor the opinion that all of the magma concerned in the 

 formation of the deep-seated bodies arrived practically in the posi- 

 tion in which we find these bodies, as basaltic magma. Where the 

 magma occurred as a small body it crystallized quickly to a diabase 

 or gabbro. This gabbroic phase may, moreover, occur as a marginal 

 phase about a larger body whose main visible portion is diorite, the 

 diorite being formed, practically in place, by the sinking out of 

 crystals from basaltic magma. In still larger bodies a more pro- 

 longed period of sinking out of crystals has left the main visible 

 portion a quartz diorite or a granodiorite, again formed in place, 

 with perhaps a border phase of diorite where the process has been 

 somewhat restricted. The magma may, of course, be injected into 

 the surrounding rocks at any late stage and form there small 

 dykes, etc., of the more acid types, but this fact should not be 

 allowed to obscure the plain tendency of the acid rocks to occur 

 principally as large masses and of the basic rocks to occur exclusively 

 as relatively small masses. 



Since the granodiorite magma comes into being only by virtue 

 of the very slow cooling of the larger bodies, it is still liquid long 

 after the diorite is a solid rock, and the same is true of diorite magma 

 with respect to gabbro. Such disturbances as may occur during the 

 consolidation of the complex may therefore cause the granodiorite 

 to acquire an intrusive relation against the diorite and the diorite 

 against gabbro. These relations are not to be considered as indi- 

 cating the order of arrival of magmas from the depths, the order 

 being due to a supposed primary or deep-seated differentiation. 

 The observed sequence is to be explained rather on the basis of a 



^ Am. Jour. Sci. (4), III (1897), 301. 



