Bulletin No. 311 Series<| £' ^""P" »«&J. 1I5 \D, Petrography and Mineralogy, 35 DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY CHARLES D. WALCOTT, DIRECTOR THE GREEN SCHISTS AND ASSOCIATED GRANITES AND PORPHYRIES OF RHODE ISLAND BENJAMIN K. EMERSON JOSEPH H. PERRY WASHINGTON GO VE R N M ENT PRINTING O F FICE 1907 ' ' CONTENTS. Page. Introduction............................................._..............'.. 7 Scope of paper................................... .................... 7 Previous work in the region............ I............................. 8 Table of formations..-..........-.......-..........-...:.........-... 8 Pre-Cambrian rocks...................................................... 9 Northbridge gneiss...........'........................................ 9 Cambrian rocks.......................................................... 10 Subdivisions and nomenclature......_-................-............... 10 Structure............................................................ 11 Grafton quartsdte..-.................--.....--..---..---.............. 12 General features ..............................._................... 12 Albion schist member............................................ 12 Marlboro formation................................................... .13 The green schists........................{\....................... 13 General characters............................................ 13 Varieties of the schist........................................ 14 Epidote-chlorite schist.................................... 14 Actinolite quartzite....................................... 15 Chlorite-actinolite schist........... ...................... 15 Hornblende schist.............. .......................... 15 Smithfield limestone member.................v.................... 16 General features.............................................. 16 Detailed descriptions of beds.................................. 18 Ores of copper, manganese, and iron....................... 18 Nonoccurrence of il vaite in Rhode Island.................. 19 Inclusion of limestone in granite porphyry................. 21 Harris bed at Lime Rock ................................. 22 Dexter quarry............................................ 24 Beds west of Providence and Pawtucket ................... 25 Cranston bed ............................................. 25 Attleboro bed............................................ 26 Beds in Woonsocket...................................... 26 Contact rocks on the Milford granite................................... 26 General features of the metamorphism ............................. 26 Contact rocks derived from the limestone .......................... 28 Trernolite rock and serpentine................................. 28 Tremolitic serpentine......................................... 28 Steatite...........................:........ ..................^ 28 Garnet rock.................................................. 29 Contact rocks derived from the green schists........................ 29 Contact effect of the Milford granite on inclosed Cambrian schists at the Saylesville granite quarry .........:......................... 31 Contact rock from the Grafton quartzite ........................... 33 Age relations......................................................... 33 3 CONTENTS. Page. Carboniferous rocks. i..................................................... 37 Scope of the discussion................................................ 37 Carboniferous boundary .............................................. 37 Carboniferous conglomerate ................................. ^......... 38 Character and distribution ........................................ 38 Outliers of Carboniferous conglomerate on the breccia of the East Greenwich group....................'..........................._... Carboniferous shales and schists....................................... Chloritoid (Masonite) in the metamorphosed Carboniferous conglomerate and phyllite ...;................................................... 42 Igneous rocks. ...................................^....................... 44 Igneous rocks of Cambrian age...............................:........ 44 Diorite .......................................................... 44 Igneous rocks of post-Cambrian (pre-Carboniferous) age.......... ...... 45 Milforcl granite................................................... 45 General description........................................... 45 The dark contact granite...................................... 46 Dike rocks....................................................... 47 Aplite . ^..................................................... 47 Pyritous aplite............................................... 48 Basic eruptives................................................... 48 Gabbro ...................................................... 48 Odinite...................................................... 49 Igneous rocks of pre-Carboniferous or early Carboniferous age:......... 51 Subdivisions ..................................................... 51 Quincy granitic group...... ....................................... 51 Distribution and general character............................. 51 Petrographic descriptions..................................... 52 Granite porphyry............... J....................'.... 52 Eiebeckite porphyry...................................... 53 Hornblende granite....................................... 55 Microgranite. ........................'.................... 55 Vein quartz at Diamond Hill.............................. 56 "Warnsutta group" of Woodworth............................ 56 East Greenwich group ....... ............:........................ 58 Distribution and general features.............................. 58 Petrographical descriptions.................................... 59 Granite and granite porphyry ............................. 59 Basic border of the granite porphyry ...................... 59 Blue-quartz-microcline porphyry .......................... 60 Microgranite............................................. 62 Microgranite or graphic microgranite ...................... 63 Dikes of microgranitic and micrographic texture............ 63 Microclinite.............................................. 64 Graphic microgranite breccia with granite-porphyry cement. 64 Chemical relations of the Carboniferous igneous rocks............... 65 Besume showing relation of the porphyries to one another and to the Car­ boniferous sedimentary rocks ....................................... 67 Igneous rocks of post-Carboniferous age. ............................... 70 Olivine-diabase porphyry ......................................... 70 Porphyritic diabase........................-...---.---.-.-----.---- .71 Index ................................................................... 73 ILLUSTRATIONS. Page. PLATE I. Map of the crystalline rocks in the vicinity of Providence and Nar- ragansett Bay, Rhode Island................................ 10 II. Porphyritic green schist....... .^.................................. 30 FIG. 1. Calcite crystal from the Harris bed at Lime Rock.................... 23 2. Calcite crystal from the Harris bed at Lime Rock..................... 24 3. Calcite crystal of the square type from the Harris bed at Lime Rock.. 24 4. Map showing distribution of metamorphosed Carboniferous rocks.... 35 5. Crystals of feldspar embedded in quartz groundmass................ 61 6. Microcline Baveno twin from quartz porphyry south of Spencer Hill.. 61 5 THE GREEN SCHISTS AND ASSOCIATED GRANITES AND PORPHYRIES OF RHODE ISLAND. Bv B. K. EMERSON and J. H. PERRY. INTRODUCTION. SCOPE OF PAPER. A great mass of coarse, porphyritic granitoid gneiss occupies the towns of Sutton, Northbridge, and Douglas, in Massachusetts, and extends southward across western Rhode Island to Kingston, on the Sound, just west of the territory to be described in this bulletin. A series of quartzites and hornblendic rocks wraps round the northern part of this mass, resting on it unconformably with a very sinuous boundary. These two groups of rocks the first called the North- bridge gneiss, the second made up of the Grafton quartzite and the Marlboro formation may be assigned with considerable probability to the pre-Cambrian and Cambrian, respectively. The continuity of the Cambrian rocks is greatly interrupted in Massachusetts by broad areas of intruded granite, and farther south, in the region here studied, only broad, isolated patches of the sedi­ mentary rocks appear, like ice floes on a pond, in a confluent expanse of various eruptives, which differ in age and in lithologic character. This paper is devoted principally to a discussion of such of these interesting Cambrian remnants as lie within the limits of the State of Rhode Jsland and to a description of the associated eruptives, espe­ cially the remarkable series of very fresh porphyritic rocks that extend westward from the town of East Greenwich. The field work occu­ pied three summers and covered the crystalline area along the western border of the Carboniferous rocks in the Providence and Narragansett Bay quadrangles. The work was undertaken for the purpose of pub­ lication in folios of the Geologic Atlas of the United States, but as the publication of the folios has been deferred and as the results of the survey are needed in the explanation of the structure of the territory adjacent on the north, in which work is now in progress, they are here presented in the form of a