WHITE: PETROGRAPHY OF THE BOSTON BASIN. 
139 
colored spherulites above referred to may have arisen from the alter¬ 
ation of biotite with the dark staining of iron ores. 
Haworth (’91, p. 5, PL 4) figures almost identical flow texture in 
quartz porphyries from Missouri. 
On the summit of Pine Hill the banded texture appears. The 
rock is a devitrihed glass with more or less contorted bands of 
darker jaspery aporhyolite with few pink feldspar phenocrysts 
interbanded with lithoidal felsite of a reddish color. The latter is a 
fine breccia ot fragments less than 4 mm. in diameter. The two 
kinds of texture are arranged in alternating bands as shown on 
weathered surfaces. This banding differs from the ordinary flow 
texture in having the lines almost free from undulations. Some¬ 
times the layers are very thin and close together, as would be the 
case around the edge of a lava sheet where the liquid flow had 
extended to the margins of previously formed flows. 4STo character 
of rhyolites is more marked in many places than this banded text¬ 
ure. Devitrification in many cases has considerably destroyed 
these lines, leaving in some of the layers the spheroidal globules. 
The general color of the rock is reddish black, the alternating 
finer bands being nearly black. Rocks of very similar texture 
occur at South Mountain, Penn., as described by G. H. Williams 
(’94) and F. Bascom (’96). Microscopically the composition and 
texture of these banded aporhyolites do not differ materially from 
the other fluidal aporhyolites already described. 
Upon Pine Hill there also occurs, near the most westerly quarry, 
porphyritic aporhyolite with a jasj3ery ground mass, the feldspars up 
to 1.5 cm. diameter, with the OP face prominent, flesh-colored, and 
weathering rather conspicuously. Carlsbad twinning is often shown. 
Smoky quartz forms particles up to 3-7 mm. in diameter. It is very 
fresh and glassy. Except for its black instead of red color, this rock 
resembles the quartz porphyries of the Lake Superior Copper Dis¬ 
trict (Irving, ’83, and Van Hise, ’92). It varies to a much more 
compact gray felsitic rock with a true devitrihed glassy base and 
some tendency to flow structure, having a lithoidal, conclioidal 
fracture. The phenocrysts are then small and hardly distinguish¬ 
able. 
Upon the most westerly of the Broken Hills there occurs an 
irregular two-foot band of a handsome bluish green rock with a very 
uniformly crystalline feldspathic texture, without distinction of 
ground mass and phenocrysts, through which are distributed small 
