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University of California Publications in Geology 



[Vol. 8 



however, to express the view that on a priori grounds the existence 

 of a floor is more probable than its non-existence. 



It should be further noted that departure from the simple con- 

 cordant relations displayed by typical small laccoliths is to be ex- 

 pected when the scale of the intrusive act is large. The roof becomes 

 then relatively weak and more susceptible to foundering, faulting and 

 stoping. The quantity of heat is larger and is more prolonged in its 

 effect. And these conditions hold for large laccoliths, whether they 

 be injected into undisturbed or disturbed strata. I know of no limits 

 to the size of laccoliths that have been discovered or formulated ; and 

 it seems to me not improbable that many large intrusive masses 

 which are commonly called batholiths are laceolithic in their mode 

 of development. The mere fact that an intrusive mass is large, or 

 that it has a transgressive relation to the remnants of its roof, is cer- 

 tainly no proof that it is a batholith. 



Batholith. — The nature of batholiths is much more obscure than 

 that of laccoliths, their genesis has not been so satisfactorily deter- 

 mined and their structural relations are more difficult to define. A 

 batholith is sometimes referred to as bottomless in contradistinction 

 to the laccolith. This characterization does not, however, aid in 

 clarifying our ideas upon the subject ; since, of course, every batho- 

 lith considered as a molten, liquid mass has a bottom. The bottom 

 is so deep that in no case has it been revealed to observation by 

 erosion. Our only knowledge of it is, therefore, inferential and un- 

 certain. The immense size of many batholiths, as well as their rela- 

 tions to adjacent portions of the earth's crust, limits, however, the 

 possibilities as to their genesis. Just as we get our simplest and 

 clearest ideas of laccoliths from a consideration of the smallest ex- 

 amples of the type, so we may get our most unavoidable conclusions 

 as to the nature of batholiths from a consideration of the largest. With- 

 out going into details it may be asserted as a necessary outcome of 

 reflection upon the general relations and dimensions of large batho- 

 liths that they develop in situ by the passage of a previously solid 

 portion of the earth's crust into a molten condition. In so far as 

 these molten bodies may be said to migrate within the earth's crust, 

 that migration, in its essential features, is effected not by injection, 

 but by a process of enlargement involving the incorporation of the 

 surrounding crust within the molten mass. In so far as offshoots 

 from the mass are injected into the surrounding crust these become 

 dykes, sills, laccoliths, etc. Batholiths are thus fundamentally dis- 



