Diller.] 362 : [January 21, 
The third argument we have noticed in favor of the sedimentary 
origin of the felsites, is drawn from the banding so prominent in 
some of the silicious felsites. There are two theories propounded to 
explain the origin of the banding. Those who believe that the fel- 
sites are sedimentary rocks, say that the banding represents the 
original bedding of the rock. Those who believe that the felsites 
of this region are truly eruptive rocks, say that the banding was pro- 
duced by the flowing of the molten rock material at the time of its 
eruption. That these lines are of igneous origin is proved by a 
number of facts. 
The banding is the same as that seen in rhyolites, whose igneous 
origin is undoubted. The similarity found by comparing the banded 
felsites with the banded glassy slag of a furnace, strongly inclines a 
person to refer the structure of both to the same general cause, 
although the particular conditions under which they were formed 
may differ widely. 
On the west side of Marblehead Neck there are a number of 
dikes of banded felsite in granite. The banding is distinctly 
marked, and exactly like much of the banding seen elsewhere. In 
some of the dikes the banding is most prominent along their sides, 
following the irregularities of their walls; but in other dikes the 
banding extends throughout the dike, and is always parallel to its 
walls. 
Half a mile north-west of the northern end of the pond near 
Saugus, is a high hill on which the junction of the felsite and granite 
can be seen. The felsite is finely banded, and the bands are parallel 
to the line of contact between the two rocks. 
On the east side of Marblehead Neck a large piece of granite is 
enveloped by felsite the bands of which are parallel to the sides of 
the enveloped fragment. The bands in the dikes of felsite, along 
lines of contact with granite, and encircling enclosed masses of 
rock, must have been produced by motion when the felsitic matter 
was in a fluid state. 
In order to compare the banding in felsites with lines of sedimen- 
tation, several thin sections were made of Cambridge slate at right 
angles to the plane of stratification. The sedimentary character of 
the slates is very evident under the microscope, and differs very 
markedly from the fluidal structure of the felsites. The groundmass 
of the latter flowed around and enveloped the porphyritic crystals 
which it contains. This peculiarity is sometimes visible without the 
