BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 6, pp. 389-422, PLS. 24-26 APRIL 17, 1895 



HIGHWOOI) MOUNTAINS OF MONTANA* 



r,\ WALTKR ir. WKKI) AND LOUIS V. PIRSSON 



{Presented before the Socletij Deeemher 27\ ISO4) 



CONTENTS 



Page 



Part I. — Geolo<]:y of the Ilijjlnvood niountains 390 



Situation 390 



Geography ami topographic characteristics 391 



General geologic structure 392 



The sedimentary platform 393 



Igneous rocks 395 



General charactei-s 395 



South and Ilighwood cores 396 



East core 396 



Shonkin core 397 



Arnoux core 398 



Palisade butte 398 



The Shonkin Sag laccolite 399 



Geologic history 499 



Part II.— Square butte and its remarkable ditferentiation zone 400 



Introductory 400 



Historical 400 



General descrii)tion of Square butte 400 



Its laccolitic origin 401 



Lower zone of dark hoodoos 402 



Upper zone of white rock 404 



Origin of the platy parting 405 



The white band 405 



Diagrammatic section of Square butte 407 



Summary < )f lielil results 407 



Petrography of Scjuare butte 407 



Characteristics and minerals of the dark rock 407 



Megascopic and nncroscopic 407 



Apatite 408 



Olivine 409 



Biotite 409 



Pyi-oxene 410 



Orthoclase 411 



•Published by permisHioD of the Director of tho United Suites Geological Survey. 

 LVI-Bui.1.. Geol, 80c. Am.. Vol. 6. 1894. (389) 



