1880.] 363 [Diller. 
aid of a microscope. The best example we have seen is in the 
reddish felsite at its junction with the black felsite on the east side of 
Marblehead Neck. The whole groundmass of the felsites must once 
have been in a viscous state, and flowed around the porphyritically 
enclosed crystals, pebbles, or anything it happened to pick up in its 
course. ‘The lenticular forms thus produced about pebbles, have led 
some observers to conclude that a part of the banding in the felsites 
has resulted from a peculiar change in the pebbles of the conglom- 
erate from which the felsite is supposed by them to have been 
derived. 
The banding in the felsites is undoubtedly igneous, and instead of 
being an argument in favor of their sedimentary origin, proves their 
eruptive character. I shall notice, when considering the breccia, the 
argument drawn from the passage of conglomerate and sandstone into 
compact felsite to prove the sedimentary origin of the felsites. 
The felsites of this region are not all of the same age. This fact 
has already been noticed by Mr. M. E. Wadsworth, who discovered 
a distinct junction between two felsites on the east side of Marble- 
head Neck.! Following the shore about two hundred yards north of 
the junction observed by Mr. Wadsworth, another line of contact 
may be seen between the same two felsites. The one felsite is red- 
dish and coarsely porphyritic, the other is black and sparingly 
porphyritic. The junction last mentioned is more distinct than the 
first, and enables us to determine the relative age of the two rocks. 
In both felsites fluidal lines were produced in the flowing mass at 
the time of its eruption. In the reddish felsite these lines are 
parallel to the line of contact between the two rocks, and in the other 
they are not, proving that the reddish felsite is the one that flowed 
along the line of contact, and must be the younger of the two. 
Other junctions have been noticed in different places, but they have 
not yet been explored. 
The “ basic felsites” form the mass of the Malden Highlands and 
extend northward into Melrose. They may occur at other places, 
but I have not yet found time to trace their boundaries. The 
relation of the “basic felsites’”’ to the felsites has not been fully 
determined. Their relation to the breccia will be considered later. 
1 See paper referred to in note 3, ante p. 359. 
