GENERAL PETROLOGY OF SQUARE BUTTE. 421 



originall}' homogeneous, but became of different composition on its outer 

 margin ])y fusion and absorption of the country rocks with which it came 

 in contact. 



Whether this is ever so or not is fairly a matter for argument. That 

 such a process cannot, liowever, be appealed to as a general explanation 

 is clearly shown at Square butte, where the outer margin, as already 

 shown, is much more basic than the interior, and yet the magma has 

 been intruded into sandstones — that is, rocks much more acid than the 

 original magma. 



The singular white band which lias ])een previously described as oc- 

 curring on the south side of S([uare butte presents on a small scale the 

 same process of dit^erentiation between the syenite and shonkinite. We 

 believe that it represents what may l)e called a residual differentiation — 

 that is, that after the main process had already taken place and the outer 

 margins of the laccolitic cavity were tilled with that magma which was 

 later going to cool and crystallize into shonkinite this further differentia- 

 tion took place in the shonkinite fluid. 



The latter, probably owing to increasing viscosity, was not able to 

 permit the white band fluid to pass in b}^ diffusion to the main body 

 of the syenite and it therefore remained parallel to the transition zone of 

 the two principal masses. 



It will be noticed that a section passing from the center to the south 

 of Square butte passes twice through white feldspathic and twice through 

 dark augiticrock, if we take the white band into consideration. Fur- 

 ther, that these various layers have a concentric arrangement with respect 

 to each other, and hence one sees that Square butte presents on a huge 

 scale a rude parallel to those spheroidal masses which sometimes occur 

 in granites and diorites, and which are often remarkable for the regular 

 concentric arrangement of spherical shells of varying composition. 



Biickstrom* has sought to explain certain cases of such spheroidal 

 masses as portions of a partial magma separated out in the liquid state 

 from a mother liquor, in which, ))y sinking temperature, they are no 

 longer soluble. 



Biickstnim has exi)and(jd this idea and sought a general explanation f 

 for the differentiation of igneous magmas in a process of •' li<| nation," by 

 which is meant that an originally homogeneous magma l)y sinking tem- 

 perature V>ecomes unstabh.' and sej)arates into two or more fluids which 

 are insoluble in each other — that is, non-miscible. It seems to us tliat 

 the concentric arrangement of parts and the clear and sharp line of divis- 

 ion between them at Square butte point very favorably to this view as 



•Geol. Foren. F6rh., Stockholm, Bd. IG, 1894, p. 128. 

 t Jour, of Geol., Chicago, vol. i, 1893, p. 773. 



LX— Bum.. Geol. Sor. Am., Vol. r,, isoi. 



