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University of California Publications in Geology [Vol. 8 



which the genesis of such a mass as that at Butte bears to this doc- 

 trine. If the intrusive be in fact a batholith, then the difficulty of 

 imagining any other source for the mineralizing waters drives us to 

 the view that these must come from the magma in the depths ; but 

 if it be laccolithic in genesis and structure the difficulty vanishes, 

 and the way is open for the recognition of meteoric waters as the 

 real mineralizing agent. 



The explanation of the failure to establish the batholithic char- 

 acter of the Boulder mass is doubtless to be found in the vagueness 

 of our ideas as to the nature of batholiths, and in the practice, too 

 often exemplified, of regarding as a batholith any large intrusive 

 mass the laccolithic characters of which are not simple and self- 

 evident. The practice is, of course, due to the temporarily imperfect 

 state of our knowledge of the criteria necessary for discrimination, 

 and in itself does no harm so long as we are aware of the loose and 

 conjectural sense in which the term batholith is used. Thus Harker 7 

 remarks : ' ' We have to recognize many extensive masses of plutonic 

 rocks which have been only partially exposed by erosion, without 

 revealing unequivocal evidence of their true geological relations. 

 These we may conveniently call batholites, using this term of Suess in 

 a purely descriptive sense, without implication of the various theo- 

 retical views which he has at different times attached to it." But 

 when it appears that important consequences, such as the doctrine 

 of magmatic waters, flow from the assumption of batholithic genesis 

 and structure, then it behooves us to realize at least upon what 

 slender foundations that assumption rests. 



DISTINCTION BETWEEN LACCOLITH AND BATHOLITH 



Laccolith. — To clear the way for an attempt in this direction in 

 the case of the Boulder mass it may be well to state here the funda- 

 mental distinction between a laccolith and a batholith. If our notions 

 of batholiths are vague, the current conception of laccoliths as de- 

 rived from definitions and diagrams is perhaps too narrow and 

 restricted to cover all cases of real laccolithic relationships. The 

 best summary of the various forms of intrusions of the laccolithic 

 type is that given by Harker. 8 



7 The Natural History of Igneous Eocks (New York, 1909), p. 82. 

 s Op. cit., p. 64 et seq. 



