116 GEOLOGICAL AND MINERALOGICAL NOTES. 



composed of feldspars and feebly polarizing nepheline in 

 a nearly complete felting of segirine crystals and grains, 

 some of which sink to the finest dust. These segirine 

 grains are so abundant in the feldspars of the ground-mass 

 that the specific gravity of the feldspar in the rock pow- 

 der, even after passing through the 100 sieve, could not 

 be clearly made out, but with the inclusions of eegirine it 

 was as low as 2.59. This rock powder gelatinized readily 

 with acid and, upon evaporation, an abundance of gypsum 

 crystals appeared, thus characterizing some of the minerals 

 in the ground-mass as belonging to the hauyne group. In 

 a communication received at a late date (June 17), from 

 Prof. H. Rosenbusch, in relation to this rock he says : 

 "Specimen No. 4 is a very good representation of the dyke 

 rocks which I have called tinguaite. Phenocrysts of 

 orthoclase in scarce quantity are disseminated in a holo- 

 crystalline mass of feldspar, nepheline and augite. I feel 

 very sure there may be some lucite in it, but I did not 

 succeed in proving it until to-day." The letter is dated 

 June 6, 1893. With this determination the phonolite dyke 

 rock would, therefore, be a lucite-tinguaite. 



The microscopical structure of the typical augite-syenite 

 from various outcrops is as follows : Thin sections pre- 

 pared from specimens collected in an old quarry on the W. 

 D. Pickman estate at Beverly Cove ; numerous large porphy- 

 ritic crystals of microcline-microperthite, 1 some multiple 

 twinned plagioclase, probably labradorite, much orthoclase, 

 augite in two forms, one in large ragged crystals, and the 

 other in long needle-shaped crystals enclosed in the feld- 

 spars as microliths, numerous small ragged crystals of 

 gegirine, some brown hornblende, red biotite in large 



1 This form of feldspar is characteristic of Professor Brogger's microline-mi- 

 croperthite in the augite-syenite rocks of Norway. Biogger, Min. der Syenite Py., 

 p. 627. 



