Diller.] 358 - [January 21, 
FELSITE 
We have thus far used the term felsite in a general sense to in- 
clude all those rocks which in this region have been called compact 
feldspar, petrosilex, porphyry, felsite porphyry, felsite, banded felsite, 
non-porphyritic felsite, hornstone, etc. 
Mr. Wadsworth has shown that the felsitic rocks of this region may 
be divided into two groups. One group, owing to its high percentage 
of silica and general vitreous characters is related to the rhyo- 
lites, the other group having a lower percentage of silica 
and more basic characters, is related to the andesites or 
trachytes. The felsitic rocks of this region have not been studied 
thoroughly enough to admit of any but a provisional classification. 
For the purpose of facilitating description I will call those related 
to the rhyolites, simply, felsites, and the other group related to the 
andesites and trachytes, I will include under the name basic felsites. 
The felsites are a fine-grained mixture of quartz and feldspar. 
The mixture is very intimate and the feldspar largely predominates. 
Orthoclase and plagioclase are both present. The former is most 
abundant, but plagioclase sometimes constitutes a considerable portion 
of the rock. Most of the felsite has macroscopic particles of quartz, 
although they are usually not nearly so prominent as the porphyritic 
crystals of feldspar. The felsites vary greatly in external appearance, 
but are uniformly compact and massive, showing no trace of stratifi- 
cation or the fragmental structure which characterize clastic rocks. 
The microscope frequently reveals a well marked fluidal structure in 
the groundmass of the felsites. This structure, so common among the 
true felsites of this region, has not been, and in all probability never 
will be, observed in any rock belonging to the stratified group. 
The felsites are probably more abundant than the “ basic felsites.’’ 
They occupy the greater portion of Marblehead Neck, and are found 
also in Lynn, Saugus, Melrose, Malden and Medford. 
Great care has been taken while determining the boundaries of the 
felsites to discover their relations to the granite and stratified group. 
Along the east side of Marblehead Neck the junction of the felsite 
and granite can be distinctly seen, and the felsite here contains in- 
clusions of granite around which it has flowed. Directly opposite this 
locality, on the west side of Marblehead Neck, the junction of the fel- 
site and granite can be seen in many places. Numerous dikes of fel- 
site occur in the granite at this place, and some of them show dis- 
tinct banding like that seen in the so-called banded felsites. 
