DEPARTMENT OF THE INTERIOR ' JOHN BARTON PAYNE, Secretary UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, Director Bulletin 704 GEOLOGY OF THE IGNEOUS ROCKS OF ESSEX COUNTY, MASSACHUSETTS BY CHARLES H. CLAPP WASHINGTON GOVERNMENT PRINTING OFFICE 1921 ADDITIONAL COPIES ' OF THIS PUBLICATION MAY BE PROCURED FROM THE SUPERINTENDENT OF DOCUMENTS GOVERNMENT PRINTING OFFICE * WASHINGTON, D. C. AT 30 CENTS PER COPY CONTENTS. Page. Introduction.............................................................. 9 Location and geologic interest of the area. 1.............................. 9 .', Object of the report...:............................................... 9 Previous work.................................................;....... 10 t Field work........................................................... 11 ! Acknowledgments...................................................... 11 I General summary and conclusions regarding the igneous rocks................. 11 i Regional geology........................................................... 15 j Geology of Essex County.................................................. 16 ( Metamorphic and sedimentary rocks.................................... 16 Cambrian and Algonkian (?) metamorphic rocks..................... 17 Distribution.................................................. 17 Lithologic character........................................... 17 . ' Quartzites, slates, and schists............................... 17 , Marbles and calcareous rocks............................... 17 ' Quartz-hornblende schists................................. 18 "Hornfels "................................................ 18 / - Age and structural relations.................................... 18 / Serpentine at Lynnfield........................................... 19 Merrimack quartzite............................................... 19 Distribution.................................................. 19 ' Lithologic character........................................... 19 Age and structural relations..................................... 19 Surficial deposits.................................................. 20 Igneous rocks......................................................... 21 Subjacent or batholithic rocks...................................... 21 Subalkaline group................:............................. 21 Salem gabbro-diorite....................................... 21 Distribution.......................................... 21 Lithologic character.......'............................ 21 Alteration............................................ 22 Structural relations..................................... 22 Newburyport quartz diorite................................ 23 Distribution.......................................... 23 Lithologic character...............................:... 23 Structural relations.-.................................... 23 ' Dedham granodiorite...................................... 24 Distribution.......................................... 24 Lithologic character................................... 24 Alteration............................................ 24 Structural relations..................................... 24 Alkaline group.......................... '. ..................... 25 Quincy granite............................................. 25 Distribution............................................ 25 Lithologic character................................... 26 Squam granite. ........<S................................... 26; . 3 4 CONTENTS. Geology of Essex County Continued. - Igneous rocks Continued. Subjacent or batholithic rocks Continued. Alkaline group Continued. Page. Syenitic rocks............................................ 27 Distribution.......................................... 27 Lithologic character................................... 27 Alteration of Quincy granite and the syenitic rocks.......... 27 Andover granite........................................... 27 Distribution........ 1................................. 27 Lithologic character................................... 28 Alteration............................................ 28 Contact relations...................................... -29 Correlation of alkaline rocks................................ 29 Gabbro associated with Dracut diorite...................... 29 Volcanic rocks..................................................... 30 Lynn volcanics................................................ 30 Distribution....................!......................... 30 Lithologic character....................................... 30 Structural relations......................................... 31 Injected rocks...................................................... 31 Laccoliths and sheets.......................................... 31 Gabbro and diabase at Nahant............................. 31 Lithologic character................................... 31 Structural relations..................................... 32 Correlation........................................... 33 Dikes..................................................... 33 Historical geology...................................................... 33 Structure.............................................................. 34 Geology of the special area ................................................ 35 Metamorphic and sedimentary formations.................................. v 35 Igneous rocks......................................................... 36 Subalkaline group ................................................. 36 Salem gabbro-diorite. .......................................... 36 Distribution.............................................. 36 Lithologic character....................................... 37 Normal rock. ........................................... 37 Variations............................................ 37 Alteration............................................ 39 Contact metamorphism and hybridism.................. 39 Chemistry and classification................................ 40 Newburyport quartz diorite.................................... 42 Distribution.............................................. 42 Lithologic character....................................... 43 Dedham granodiorite.......................................... 44 Distribution.............................................. 44 Lithologic character....................................... 45 Felsic inclusions.......................................... 46 Chemistry and classification................................ 47 Alteration and metamorphism............................... 48 Aplites................................................... 48 Basic dikes.................................................... 49 Magmatic relations............................................. 49 Structural relations........................"..................... 50 Differentiation of subalkaline magmas.......................... 53 CONTENTS. 5 Geology of the special area Continued. Igneous rocks Continued. Page. Alkaline group................................................ ^... 58 Lynn volcanics................................................ 58 Distribution.............................................. 58 Qiiartz keratophyre. ....................................... 58 Normal rock........................................... 58 Alteration............................................ 59 Chemistry and classification............................ 60 Trachyte................................................... 61 Dacite .........=...........................:.............,.. 61 Andesite................................................. 61 Textural varieties......................................... 62 Perlitic and spherulitic varieties........................ 62 Taxitic varieties....................................... 62 Agglomerates and breccias.................................. 63 General character..................................... 63 Breccias............................................... 63 Agglomerates of Vinegar Hill............................ 64 Basal agglomerates.................................... 64 Breccia dikes.......................................... 65 Dike rocks................................................: 66 General character..................................... 66 Bostonite or keratophyre.............................. 67 Metamorphism............................................. 68 Jointing............................................... 68 Silicification........................................... 68 1 ' Cherts".............................................. 69 Structural relations of Lynn volcanics...................... 69 Plutonic rocks................................................ 71. Quincy granite............................................. 71 Name................................................ 71 Distribution.............................................. 71 Lithologic character................................... 72 Composition....................................... 72 Color.............................................. 72 Mineral association................................ 73 Texture........................................... 73 Microperthite........ ............. I............... 73 Albite............................................ 75 Quartz............................................ 75 Katophorite........................................ 76 Hedenbergite..................................... 76 Colorless pyroxene (?).............................. 76 Basaltic hornblende................................