DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, DIRECTOR BULLETIN A92 THE GABBROS AND ASSOCIATED ROCKSl AT PRESTON, CONNECTICUT BY G. F. LOUGHLIN WASHINGTON GOVERNMENT PRINTING OFFICE 1912 CONTENTS. Page. Preface................................................................... 9 Introduction:............................................................. 11 Location.............................................................. 11 Topography........................................................... 12 Hills...........,................................:................. 12 Valleys........................................................... 12 Drainage. .......................................--.-.-...- ------- 13 Lakes and ponds.................................................. 14 Historical review....................................................... 14 Areal geology.............................................................. 15* Roeks of sedimentary origin............................................ 16; Types............................................................ 16 Quartzite.......................................................... 16 Quartz-biotite schist. ............................................. 17 Hornblende schist. ............................................... 18 Black pseudoporphyritic schist (kinzigite).......................... 20 Dolomite.......................................................... 20 Igneous rocks......................................................... 21 Preston gabbro.................................................... 21 Sterling granite gneiss.............................................. 23 Lantern Hill quartz.................................................... 24 Structural geology........................................................ 24 Schistosity............................................................ 24 Folding.............................................................. 25 Undulations...................................................... 25 Minor folds:.....................................'................. 26 Major folds....................................................... 28 Arrangement of folds.............................................. 29 Origin of folds.................................................... 30 Effect of the gabbro intrusion.................................. 30 Effect of the granite intrusion................................... 32 Summary..................................................... 32 Igneous intrusions.................................................... 33 Preston gabbro.................................................... 33 Sterling granite gneiss.............................................. 35 Intrusive areas................................................ 35 Porphyritic granite gneiss areas............................ 35 Alaskite areas............................................. 36 Normal granite gneiss areas................................. 36 Minor sheets and dikes..................................... 37 Structural interrelations of the granite intrusions............. 37 Method of intrusion............................................ 3& 3 4 CONTENTS. Structural geology Continued. Page. Metamorphic deformation............................................. 38 Regional metamorphism of the Preston gabbro......I............ T.. 38 Contract metamorphism. due to the Preston gabbro................... 39 Regional metamorphism of the Sterling granite gneiss................ 43 Contact metamorphism due to the Sterling granite gneiss. ........... 43 Joints................................................................ 44 Distribution and directions......................................... 44 Origin of joints................................................... 47 Relation of joints to stream courses.................................. 48 Principal valleys............................................... 48 Major tributary valleys.......................................... 49 Minor tributary valleys......................................... 50 Conclusions................................................... 50 Faulting............................................................. 50 Character and distribution......................................... 50 Ashwillet intrusion fault........................................... 51 Broad Brook fault.................................................. 52 Minor faulting..................................................... 52 Veins................................................................ 53 Quartz veins....................................................... 53 Chlorite and asbestos veins in gabbro................................ 53 Special erosionalfeatures.............................................. 54 Pitted and warty surface of gabbro............:..................... 54 Dissolution holes................................................... 54 Historical geology......................................................... 57 Petrography............................................. r ................ 58 Metamorphic sedimentary rocks......................................... 58 Quartzite.......................................................... 58 Megascopic characters......................................... 58 Microscopic characters..............................;.......... 58 Variations and similar rock from other horizons.................. 59 Quartz-biotite schist. ............................................. 59 Megascopic characters......................................... 59 Microscopic characters......................................... 59 Allanite....................................................... 60 Variations...................................... J.............. 60 Epidote gneiss................................................. 61 Hornblende schist................................................. 62 Megascopic characters......................................... 62 Microscopic characters. ........................................ 63 Variations...............................'..................... 63 Basic slate........................................................ 63 Megascopic characters......................................... 63 Microscopic characters......................................... 64 Basic hornfels...................................................... 65 Hornblendic variety........................................... 65 Megascopic characters. .................................... 65 Microscopic characters..................................... 65 Pyroxenic variety................................'............. 65 Megascopic characters..................................... 65 Microscopic characters..................................... 65 Origin of the hornblende schist. ............................... 66 CONTENTS. 5 Petrography Continued. Metamorphic sedimentary rocks Continued. Page. Black pseudoporphyritic schist (kinzigite).......................... 68 Literature.................................................... 68 Megascopic characters......................................... 68 Microscopic characters......................................... 71 Chemical composition......................................... 72 Variations.................................................... 73 Origin........................................................ 74 Dolomite.......................................................... 76 Megascopic characters......................................... 76 Microscopic characters......................................... 77 Variation..................................................... 78 Preston gabbro........................................................ 78 Types............................:................................ 78 Porphyritic gabbro................................................ 79 Normal phase................................................ 79 Megascopic characters..................................... 79 Microscopic characters..................................... 80 Mineral structure.................................... 80 Diallage.............................................. 80 Secondary brown hornblende......................... 80 Biotite............................................... 84 Labradorite........................................... 84 Ilmenite...................... I..................... 87 Chlorite-magnetite aggregates......................... 88 Pyrite................................................ 88 Quartz-calcits veinlets................................... 93 Mechanical and chemical metamorphism ................... 98 Chemical composition..............................."...... 94 Classification............................................ 94 Norite phase (auvergnose-hessose).............................. 94 Megascopic characters..................................... 95 Microscopic characters..................................... 95 Mineral structure.................................... 95 Labradorite...............:........................... 95 Bronzite............................................. 95 Diallage.............................................. 96 Ilmenite and pyrite.................................. 96 Alteration products.................................. 96 Classification............................................ 96 Olivine gabbro phase......................................... 96 Megascopic characters..................................... 96 Microscopic characters....................................