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Highwood Mountains, Montana Bulletin No. 237 Series j ft ?e?criPtlv« Gwl°^ 43 , 90 \ D, Petrography and Mineralogy, 29 DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY CHARLES D. WALCOTT, DIRECTOR PETROGRAPHY AND GEOLOGY OF THE IGNEOUS ROCKS OF THE HIGHWOOD MOUNTAINS, MONTANA BY LOUIS VALENTINE PIRSSO1ST WASHINGTON GOVERNMENT PRINTING OFFICE 1905 CONTENTS. Page. Letter of transmittal. .......................^............................ 11 Introduction__._.._._.____..--.-----_--.------..--.---...---- .-.------ 13 Bibliography ___.._______. __.__._____..---.._..-------.--.-..----..-..- 14 Chapter I. Geography and history ........----.-....-..--...-.----..----- 14 Location ..__._.._.._____..---..._------- --------------------. 14 Bibliography .. ...-.-...-..----.-_.._. ...... ....................:...... 15 Topography and geography .......................:.-.------.------.- 15 History................................................................. 16 Chapter II. Geology of the igneous stocks ................................ 20 Introductory ................................................... ------ 20 Highwood Peak stock....-.-.-..........-._.....--...-.---_.-.--._... 20 Character of the border contact.-..-.-.-..-..-.--...-----..-.--.- 22 Middle Peak stock.................................................. 23 Contact phenomena. -..---.......---------. -..-..-..--------.-.-- 23 Endomorphic contact phenomena ..........^..................... 24 East Peak stock....................................................... 24 Shonkin stock ...................... ....-'---.--...--".-...-.---.-.---.- 26 Occurrence of missourite.______.--.,._._.._--_.- --_-------- 27 Arnoux stock........................................................ 28 Chapter III. Geology of the sheets, dikes, and extrusive rocks............. 30 Intrusive sheets.............--.-........-.----.-.-...-....--...-.---_. 30 Dikes.... ..-....._.____..-_.....__..._..-_. .....-.-- _....... 31 Introductory ._____._.._.:_________. ...____'_._______..____.____ 31 Radial disposition .................^........:..................... 31 Character .-.................................................-.. 32 Rocks composing the dikes ....................................... 34 Light-colored f eldspathic (salic) dikes............................ 34 Dark basaltic (femic) dikes ......... ..............--...----...., -35 Relative age of the dikes .......................................... 35 Extrusive flows and breccias ...........'............................... 36 Areal distribution ...... ........ .................................. 36 Feldspathic extrusives of the first period.......................... 37 Basaltic extrusives of the second period........................... 38 Distribution of basaltic extrusives. ........................... 39 Sources...............................-......................_... 39 Chapter IV. Geology of the laccoliths .................................... 42 Introductory ............ ......................................... .... 42 Minor laccolith of the Shonkin Sag .._..-..-.............-..-.......... 43 Major laccolith of the Shonkin Sag ................................... 43 Ends of the laccolith wall.............................. ....... 44 The laccolith rock............................................ 44 Interior of the laccolith ...................................... 45 Causes of dissection.......... .... ............................. 46 Internal structure .............................................. 46 3 4 CONTENTS. Chapter IV. Geology of the laccoliths Continued. Page. Palisade Butte.... ......................................... r ......... 48 Rock variation .............................................. 48 Laccolithic character ........................................ 49 Square Butte --.-----.I--..--..--.....................-....-.---. ...... 49 Introductory.........._................................---... 49 General description__________________________________________ 49 Laccolithic origin . ._____________._.._____.._.__._._._.__.._.__ 50 Lower zone of dark hoodoos.. .....................r.......... 50 Upper zone of white rock .................................... 51 . Origin of the platy parting................................... 52 Diagrammatic section.......-.-....---...-..--.........-. ..v 53 Chapter V. The sedimentary platform...-......--.....--.-..---.-.---.... 55 Introductory _____..__.._._._____._..._.____.___.___.___.____ 55 Cascade formation ______________.________ .______..____.._.... 55 Dakota formation............................................ 55 Colorado formation .......................................... 55 Eagle formation .................... ......................... 56 Montana formation ._-.......-.....---... --.---.-.--.-.-.---- 56 Summary. ---------................-.--..---.....---...-.-... 56 Chapter VI. Petrography ................................................. 57 Introduction........................................................... 57 Summary of Lindgren's work ....._........--...----...-..... 57 Previous work of the author. ................................. 58 Classification ........................................................ 59 Stocks and laccoliths.-.---.................-.....-.............-----. 60 Grano-pulaskose (syenite var. pulaskite) of Highwood Peak ...... 60 Megascopic characters -.---..--..---...---.--.-... .--..-..... 60 Microscopic characters.-------------- ..-..---.'-.---.------.-. 61 Chemical composition........................................ 63 Mineral composition or mode..-....------...:..---..---..-.-. 64 Classification in the new system............................... 64 Classification in prevailing systems...----....-.-.---.--...... 65 Pulaskose (sodalite-syenite) of Square Butte......-----.-..--.-.-. 66 Introductory ................................................ 66 Megascopic characters ...........................!........... 66 Texture ..................................................... 66 Microscopic characters. -....--..-....--.--..-.....-......-..... 66 Mineral composition or mode.._._._---.--------.----....---.-- 67 Chemical composition........................................ 68 Classification in prevailing systems.----..-----.--.------.. .. 69 Classification in the new system ^............................. 69 Comparison with related types -......-....-.-...--.---.-...... 70 Tracho-highwoodose (nosean-syenite) ....----.-....-----...-.---- 71 Occurrence ____.....__.._...-.__.__......_..__.__.___.._.____ 71 Megascopic characters ...........................'............ 71 Microscopic characters..-----.-------.---.--.-.-----.-.- ... 71 Chemical composition ----...--.-..-----..-..----..----------. 72 Classification in the new system......-..,........-..--..----.. 75 Classification in prevailing systems..-.--..--.--...-.-.----..- 76 Grano-shoshonose .(monzonite) of Highwood Peak .__..___.______. 76 Introductory ................................................. 76 Megascopic characters ---.-..---....---..------.--..-.-..-..- 76 CONTENTS. 5 Chapter VI. Petrography Continued. Page. Stocks and laccoliths Contintied. Grano-shoshonose (monzonite) of Highwood Peak Continued. Microscopic characters. --- _-____._._.....__ _._____....__..... 77 Behavior with acids.......................................... 78 Chemical composition..._-_-._..-_-...._._..__.......-.__.... 78 Mineral composition or mode...-.......-.-.-..-.-.-..--.....- 80 Texture ....................................................... 80 Classification in the new system............................... 81 Classification in prevailing systems. ------.-..-.--..-.-------- 82 Dikelets..................................................... 82 Contact f acies .-.-..-.--..---..-.-....----.--...-...---.-..-- 83 Fergusose (fergusite) of Arnoux stock............................ 83 Occurrence.................................................. 83 Megascopic characters -.....--....-.......-...-..-....-- --.---- 83 Microscopic characters. --.-.-..-,--.-.-.-.---..--..-.-......- 84 Chemical composition..............--..-...-..'......----.-.-. 85 Texture ..................................................... 87 Classification in the new system -......-.-....-..-.-.-.-----.. 87 Mineral composition or mode................................. 88 Classification in prevailing systems. -------.--.--.......-.---- 88 Grano-borolanose (basic syenite, shonkinitic type) of Middle Peak. 89 Megascopic characters -.-....-.-..--.-..-.......-----.--.--.. 89 Microscopic characters---....-----.----...---_-.-..---.....-- 89 Chemical composition..........-....-.--.-..'..-..-....--.--.- 91 Mineral composition or mode........................... ...... 92 Classification in the new system.............................. 93 Classification in prevailing systems --------------------------- 94 Border f acies ...............----.. .---...-----.----.-.-.--.-- 94 Borolanose (syenite) of Palisade Butte .,.........!................ 95 Borolanose (syenite) of Shonkin Sag laccolith-.____..__-...-....... 96 Shonkinose (shonkinite) of Square Butte -.....-.-------_.-----..-. 97 Megascopic characters -..--.-...---.-----.----.----.--------. 97 Microscopic characters..... .-..-----..-----..-.....--- -.----. 98 Chemical composition..........-.-....-...-.-..-....-....-.-. 102 Mineral composition or mode...--.--.---......-.-..-.-..----.- 103 Classification in the new system.....................---...... 104 Shonkinose of other Highwood localities.....-..--_.-..-.------.-- 105 Leucite-shonkinose (leucite-shonkinite) of East
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