1914] Lawson: Is the Boulder "Batholith" a Laccolith? 3 



This is a very strong argument in favor of magmatic emanations as 

 a direct agency in ore deposition. But it lacks conclusiveness in the 

 fact that the deposits in question are "distinctly later than the in- 

 trusion of the aplites, " which in turn are intruded in the quartz- 

 monzonite that forms the main mass of the Boulder "batholith." 



Similarly Sales 5 accepts the batholithie origin of the Butte 

 granite (quartz-monzonite, aplite, etc.), and holds that 



The original source of the ores at Butte was the granite (quartz-monzonite) 

 magma. Quartz-porphyry dikes formed a local closing phase of the igneous 

 activity connected with the intrusion of the parent rock, and these dikes 

 structurally and areally are in such close association with the ore deposits 

 that they appear to be a direct factor in the localization of the ores. Heated 

 waters and gases escaping from the cooling magma were the carriers of the 

 metals to their place of deposition. 



The conclusions thus reached by these investigators, as to the 

 magmatic source of the water concerned in the deposition of the 

 ores, appear to be logically necessary imder the hypothesis that the 

 intrusive mass is a batholith. It is the purpose of this paper to in- 

 quire into the evidence upon which that hypothesis rests and to 

 suggest an alternative one from which no conclusion flows as to the 

 magmatic origin of the mineralizing waters. 



VAGUENESS OF THE TERM BATHOLITH 



A review of the literature of the subject brings out the curious 

 fact that there is no exposition of the evidence which has led to 

 the general recognition of the intrusive mass at Butte as a batho- 

 lith. That it is such appears to be pure assumption. There have 

 been doubtless latent considerations in the minds of those who have 

 so freely declared it to be a batholith, which have influenced them 

 in adopting this view but these considerations have not been set 

 forth; nor has any attempt been made to discuss the matter, so far 

 as I can discover. This is the more remarkable in view of the great 

 prominence given in recent years to the doctrine of magmatic waters 

 as a prime agent in ore deposition, and of the fundamental relation 



o Ore Deposits at Butte, Montana, Bull. Am. Inst. Min. Eng., No. 80, p. 1611, 

 Aug., 1913. 



« We have, however, an able discussion of the structural relations and 

 method of intrusion of the neighboring, but much smaller, intrusive mass, at 

 Marysville, by Barrell in his monograph on the Geology of the Marysville 

 Mining District, Montana, U. S. G. S. Prof. Paper 57. Barrell 's argument that 

 this intrusion is a batholith will be considered briefly in the sequel. 



