216 The American Geologist March, i897 
Red, yellow and purple volcanics are conspicuous along the bold 
shores of the Fox Islands thoroughfare, whence they have been designa- 
ted the Thoroughfare VOLCANICS. These lavas and their corresponding 
fragmentals include three rock-types: andesites, diabase porphyries and 
quartz-porphyries. These ancient andesites vary somewhat in minera- 
logical composition and in structiire and admit of further classification 
as pyroxene-andeslte, hornblende-andesite, basaltic andesite, andesite- 
porphyry and amygdaloidal andesite. Reference will again be made to 
these rocks in the discussion of terminology. A volcanic conglomerate 
made up of rounded andesite fragments, ranging in size from that of a 
boulder several feet in diameter to that of coarse sand, is the most im- 
pjrtant of the pyroclastics. Flow breccias and tuff breccias are also 
found with gradations to tuffs of the finer volcanic dust. The ash 
structure, characteristic for deposits of finely divided glass, is displayed 
by these tine grained tuffs, and is figured by the authoi'. 
The pyroxene-andesites and andesite- porphyries ai'e the lowest mem- 
bers of the series and are immediately overlain by the volcanic conglom- 
erate, which in turn is overlain by basaltic andesite, the more acid 
hornblende andesite, acid and basic interbedded tuffs and amygdaloids. 
The limited flows of quartz porphyry are associated with the acid tuffs. 
The determination of the age of these volcanics rests upon their relation 
to the Niagara sediments. While at one locality (Ames knob) the vol- 
canics overlie, without marked unconformity, the upper members of 
the Niagara series, at another locality (Stimpson's island) some presum- 
ably Niagara sediments are interbedded with the lower members of the 
volcanic series. 
These facts justify the assumption that the volcanic activity began in 
Niagara time. No evidence of the site of the volcanic vent was ob- 
tained. The sediments, volcanic and otherwise, indicate that the area 
of volcanic action was near the sea level and underwent more or less 
frequent oscillations of level. 
Nor did volcanic activity close with the Thoroughfare series, but later 
activity is represented by the Vinal Haven acid volcanics. These acid 
volcanics occupy the northwest end of Vinal Haven and, while exhibit- 
ing few types, they are of peculiar petrographic interest and beauty. 
They furnish another illustration of the preservation, in ancient devit- 
rified acid lavas, of the structures found in modern rhyolites and obsid- 
ians. They include taxitic and spherulitic aporhyolites, with flow 
breccias and tufts of the same type. The spherulites vary in size from 
a pin-head to spheres several inches in diameter. No litbophysai were 
found. The matrix in which the spherulites occur is cryptocrystalline 
and occasionally vesicular. The presence of perlitic cracks is taken as 
an indication that the crystallization is, in part at least, the result of 
devitrification of an originally glassy matrix. 
The flow breccias illustrate still more strikingly the original character 
of a glass and in one instance, at least, subsequent devitrification has 
been very slight. This occurrence of glass in such ancient lavas is 
unique. 
