The great intrusive between Helena and Butte, Montana, generally referred to as the Boulder batholith, has so much interest from an economic point of view that its structure and deeper parts have been several times discussed.1 It seemed to the writers that this interest justified a reconnaissance by the structural methods of Hans Cloos.2 This might not only help to clarify certain aspects of the argument as to the form of the mass, but might have some application to the general theory of ore deposition.

Furthermore, the early work of the Cloos school of geologists was done on masses that were emplaced under considerable cover, whereas the Boulder mass has probably some remnants of its roof, and that roof may not have been very thick. It is probably a fair example of a batholith, the emplacement of which was accompanied by little compression of the crust, and the structures . . .

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