Edkle — Biotite-tinguaite Dike from Massachusetts. 489 



Akt. LII. — A Biotite-tinguaite Dike from Manchester by the 

 Sea, Essex County, Mass. / by Akthuk S. Eakle. 



The dike described in this paper cuts through the augite- 

 syenite of Gales rocks, 200 yards south of Gales Point, Man- 

 chester, and was discovered in July, 1896, by Mr. J. H. Sears, 

 while investigating the rocks of that vicinity. The writer has 

 not seen the dike, and all of the data regarding its occurrence, 

 and the material for the petrographical and chemical study, has 

 been very kindly supplied by Mr. Sears. "The dike is 6 

 inches wide and is exposed for 20 feet, cutting the augite- 

 syenite in a nearly horizontal position, six feet below the sur- 

 face of the mass of syenite. It is only exposed at low water, 

 as at high tide the entire mass of svenite is submerged." The 

 occurrence of this segirine dike in the immediate vicinity of 

 the tinguaite dike at Pickards Point, which was first described 

 by Sears* and lately shown to be an analcite-tinguaite by 

 Washington,! might naturally lead to the supposition that the 

 two dikes would be similar in many respects, yet both macro- 

 scopically and microscopically they are quite dissimilar rocks. 



The rock has a greenish-gray color and a slightly greasy 

 luster, like tinguaites and rocks rich in nepheline. Small 

 phenocrysts of feldspar are visible in the somewhat compact 

 groundmass, and also much magnetite, mixed with biotite, 

 occurs in brownish-black patches, giving the rock a mottled 

 appearance. The structure is compact, noncrystalline, the 

 rock resembling a phonolite, breaking with an even fracture 

 and weathering to a light gray color. 



Under the microscope the principal constituents are seen to 

 be feldspathic laths and plates with much nepheline and less 

 amounts of segirine, magnetite and biotite. Besides these 

 prominent minerals, hematite, a little sodalite, a few apatite and 

 zircon needles, and minute sections of purple fluorite are 

 present. 



The feldspar is the most abundant constituent and predomi- 

 nates in lath-shaped sections, most of which have a broken, 

 ragged appearance, due to frayed-out ends and a fibrous struc- 

 ture. This fibrous appearance is evidently the result of lamel- 

 lar intergrowths of the soda and potash feldspars, microcline 

 and albite, forming microcline-microperthite. Some of the 

 broader sections show a rather coarse intergrowth of the two 

 feldspars, giving extinctions on different parts corresponding 

 respectively to those of microcline and albite. The character- 



* J. H. Sears, Bull. Essex Inst., vol. xxv, 4, 1893. 

 f This Journal, IY, vol. vi, p. 182, 1898. 



