Weed and Pirsson — Highivood Mts. Laccoliths. 13 



without inwardly, and finally the laccolith was formed as we 

 now find it. 



On the other hand, we do not believe that the laccolith was 

 produced by successive injections of magmas of different chem- 

 ical composition which were injected into those already injected 

 and solidified, splitting them in the process. The significant, 

 orderly and regular arrangement of the different parts in con- 

 centric zones, the lack of contact phenomena between them, 

 their gradation into one another from the most basic on the 

 outside to the most acid on the inside, and the chemical rela- 

 tionships of the different types all seem to us to clearly forbid 

 such a supposition, to say nothing of the mechanical difficulties 

 involved. And also the recurrence elsewhere in this district 

 of the same rock types with similar arrangement as at Square 

 Butte, has a positive bearing on this point and is another 

 strong, if not conclusive argument against such a theory. 



It also appears to us that the theory, where acid, feldspathic, 

 intrusive masses are formed with basic border zones rich in 

 lime, iron, and magnesia, that this is due to absorption of such 

 material by the fluid magma from the surrounding rocks is not 

 applicable in this case, however, it may be elsewhere. The 

 reasons for this may be divided into three heads. 



First. From the regular and orderly arrangement of the 

 parts, it is evident that such absorption could not have taken 

 place at lower horizons with subsequent injection of the homo- 

 geneous magma into its present position. If melting and 

 absorption of the sediments took place, it did so in the place 

 where the mass now is. But in this case it is clear that the 

 magma must have had the composition of the inner, highly 

 alkaline syenite. An inspection of the section drawn on the 

 natural scale will at once show that, to produce the great body 

 of shonkinite from the syenite, there must have been melted up 

 and absorbed a vast body of surrounding rock, an effect incon- 

 ceivably too great for any body of injected molten magma to 

 perform. And in this case how should we explain the thin 

 fringing sheets, homogeneous throughout, having nearly the 

 same composition as the shonkinite, and which run out for 

 such great distances among the unchanged sandstones. 



Second. The theory demands that lime, iron, and magnesia 

 should be the elements absorbed, but the laccolith lies sur- 

 rounded by Cretaceous sandstones, composed chiefly of quartz 

 with some feldspar. Indeed, in the sedimentary formations 

 of this portion of Montana described by us in numerous 

 memoirs,* we know of no series of beds which, melted up, 



* Geology of the Castle Mountains Mining District. Bull. 139, U. S. Geol. Surv.; 

 Geology of the Judith Mountains, 18th Ann. Rep., U.S. Geol. Surv., Part III, 

 p. 437; Geology of the Little Belt Mountains, 20th Ann. Rep., U.S. Geol. Survey, 

 Part III, p. 257. 



