156 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



inconspicuously and minutely porphyritic ; the central portion of the dike shows 

 a less aphanitic texture and more conspicuous and numerous phenocrysts of 

 pinkish or green epidotized feldspar, which extinctions indicate to be a feldspar 

 between AbiAni and AbsAn 4 ; no quartz is present; the ferromagnesian constituent 

 is completely altered to actinolite or chlorite associated with epidote ; apatite crys- 

 tals are still well preserved ; epidote occurs both as a granular aggregate and in 

 good crystals; an orthophyric fabric is more or less traceable though extended 

 chloritization and epidotization often obscure the original fabric as well as the 

 constituents. 



A complete analysis was not made of this type. The silica, alumina and 



oxides were determined as follows 



SiO*. 

 Al 2 0, 



FeO. 



Border of Dike. Center of Dike. 



.... 60.00 62.8 



19.84 

 8.70 



The lime was found to be relatively low and the magnesia high. 



These determinations suggest a hornblende bearing biotite-andesite. The 



material is too altered and the analysis too incomplete for more exact determi- 

 nation . 



Diabase Dikes.— Cutting indifferently the members of the Neponset igneous 

 complex occur two systems of diabase dikes; an east-west and a north-south 



system. 



The east-west system includes very numerous dikes which vary in width from 

 an inch to seventy-five feet; they are composed of a compact, fine-grained, dark- 

 colored rock, of which the weathered surface is reddish brown, the interior gray- 

 green, the fracture conchoidal, and the fabric micro-ophitic. 



These dikes are cut by the north-south dikes, otherwise they cut all the other 

 formations and represent the most recent igneous action in the district. They are 

 characterized by a regularity of form, a parallelism of trend, and a continuity of 

 occurrence not possessed by the other intrusives, nor do they show any relation 

 in origin or composition to the other igneous materials of the basin. 



Specimens were collected from dikes ten, twenty-five, forty, and forty-five 

 feet in width ; while the grain of the rock varies with the width of the dike, all 

 of the material is finer grained than the Triassic diabase sheets and flows of 

 Connecticut and New Jersey, and averages about the same in grain as the diabase 

 of the Triassic dikes in eastern Pennsylvania. 



Petrographically the rock is extremely uniform and presents no variations 

 from the normal type of the Triassic diabase. In the hand specimen the ophitic 

 structure is somewhat obscured by the uralitization of the feldspars, an alteration 

 which is responsible for the green color of the rock. Feldspar occurs in the usual 

 isomorphic columnar crystals, but so altered that few extinction angles could 

 be secured; these correspond to those of labradorite with the composition AbaAn,; 

 a little orthoclase seems to be present in addition to the plagioclase feldspar. 



