WHITE: PETROGRAPHY OF THE BOSTON BASIN. 
137 
Lines and grains of decomposed limonite are scattered through it. 
The hornblendes as a rule are more chunky and are rounded. 
Fluidal Aporhyolite. — The fluidal aporhyolite first appears 
in contact with the presumably earlier flow of porphyritic aporhyo¬ 
lite, near the summit of Wampatuck Hill. In the field it is a 
compact, jaspery, purplish black rock with brilliant flesh-colored 
feldspars, showing chiefly the face OP, in crystals up to 3 or 5 mm. 
long, scattered through it. It also shows in places a very little 
smoky quartz in fine grains, and occasionally bundles of needles of 
yellowish tourmaline. The weathered surfaces show the contorted 
flow structure, giving that peculiar wavy, or often knotted appear¬ 
ance so common in blast furnace slags and modern volcanics. Dr. 
Wadsworth 1 first called attention to this fluidal structure, so long 
mistaken for remains of sedimentary origin in these felsites and 
porphyries of the Boston Basin, as the strongest evidence in favor of 
their eruptive origin. 2 It is caused by the flowing movements of 
the molten magma, or ancient lavas, the slight inequalities in the 
composition of which, or the presence of iron oxides, crystallites, or 
solid particles render the structure prominent only after weathering, 
or in thin sections (Iddings, ’87). By polarized light the base is 
found to be completely devitrified, giving rise to the so-called micro- 
felsitic base. The flow texture of the rock, followed by its sudden 
chilling, probably arose from the magma being so siliceous. In the 
subsequent devitrification, the silicates crystallized out, leaving the 
residual silica in the form of quartz occupying the interspaces. 
Thin sections reveal a well-marked fluidal texture of the ground 
mass, even where the rock, en masse , does not. The particles either 
assume lenticular outlines as they flow around the phenocrysts, or 
else they form narrow chains of spherulites. It is noticeable that 
these rocks are all of a purplish black color, rendered gray with 
black lines in thin sections. Rocks of precisely this character occur 
on the Cranberry Islands, Vinal Haven, Eastport, Lubec, and else¬ 
where on the coast of Maine (Shaler, ’89, Williams, ’94, Bayley, 
’95, and Smith, ’95 and ’96) ; as well as in New Brunswick, (Mat¬ 
thew,’94 and’95). As Professor Williams (’94) has pointed out, 
1 In Whitney and Wadsworth (’84, p. 429); see also Wadsworth (’79), Diller (’80 and '81, 
p. 169), and Hyatt (’76). The felsites are the rocks classed since Godon’s (’09) time as petro- 
silex. 
2 It is worth noting that, as early as 1822, Dr. Thomas Cooper wrote (’22, p. 239): “ No 
person accustomed to volcanic specimens can look at the porphyries from the neighbor¬ 
hood of Boston * * * and doubt their volcanic origin.” 
