Johnson and Warren — Geology of Rhode Island. 17 



more commonly, however, as roughly equidimensional grains 

 from 0*10 to 0*20 mm in average diameter. They were called 

 glass by Wadsworth and were -thought to be hercynite by Geo. 

 H. Williams, to whom they were submitted by Wadsworth for 

 examination. The crystals are very refractory to chemical 

 decomposition since a few were found undecomposed by sul- 

 phuric and hydrofluoric acid in some of the ferrous iron deter- 

 minations made on the rock. They are undoubtedly spinels 

 (pleonaste), an identification substantiated by the ratios 

 derived from the rock analysis given beyond. Spinel is lack- 

 ing in sections cut from some of the bowlders previously referred 

 to, but appears, however, to have been generally present in the 

 rock of the hill, and its absence in the bowlders probably indi- 

 cates only a local variation in the composition of the magma. 



The olivine. — The olivine is nearly colorless in thin section 

 with but a slight brownish or yellowish tint, and is remarkably 

 fresh throughout. Fractures are abundant, and the basal and 

 brachy-pinacoidal cleavages are better developed than with 

 ordinary olivine. Between crossed nicols the olivine is broken 

 up into a mosaic of anhedral grains, showing all gradations in 

 size from mere points up to individuals 5 mm long and 2'5 mm wide. 

 In a few instances an olivine grain has been observed with a 

 distinct crystallographic outline against the ore, but in general 

 the olivine, like the other constituents (except the spinel), are 

 conspicuously lacking in crystallographic outlines. Single 

 olivine anhedrons are everywhere to be seen enclosed in the 

 ore, and throughout the rock the latter acts always as the 

 matrix. 



The olivine anhedrons are often separated from one another 

 by strongly marked black borders, consisting of minute opaque, 

 black grains, accompanied by some that are reddish or brown- 

 ish by transmitted light, and very similar in appearance to the 

 inclusions in the olivine. These borders are commonly 

 continuous with the ore matrix. 



As previously noted, the olivine anhedrons are traversed by 

 many fractures. These often extend through several indi- 

 viduals, and more rarely form a series of rudely parallel frac- 

 tures, extending across the entire section. In the latter case 

 they are sometimes filled with granulated material of the mineral, 

 through which they pass, and are more or less discolored with 

 serpentinous matter. More commonly they are tilled with 

 the same black, finely granular material that lies between 

 the crystals. In several slides examined, numerous fractures 

 were observed, radiating outward into the surrounding olivine 

 from an ore grain. It appeared as if the olivine had been 



Am. Jour. Sci.— Fourth Series, Vol. XXV, No. 145.— January, 1908. 



9 



