Diller.] 366 [January 21, 
mentary. When the line of contact between the felsite and breccia 
is not prominently marked in nature, one of the best ways to find it 
is to make a series of microscopic sections on a line at right-angles 
to the line of contact. A study of the thin sections will enable the 
observer to tell between which two of the series the line of eontact 
will be found. Having thus limited the area of observation the 
line can be found quite readily. A considerable portion of what is 
generally considered breccia is simply felsite containing fragments 
picked up at the time of its eruption. 
The fourth argument in favor of the sedimentary origin of the 
felsites, like some of the other arguments, is not based on facts, and 
is therefore of no value. 
CONCLUSIONS. 
The stratified group does not contain, as a part of itself, a clearly 
interstratified rock which is properly called felsite. 
The stratified group of Melrose and Saugus is cut by the granites 
of the same region, and must be older than the granites. The 
distinct line of contact between the granites and the stratified 
group proves that they are distinct formations. Facts already stated 
are undoubted evidence that the granites are eruptive. 
The distinct line of contact between the felsites and the granites, 
together with the dikes of the former in the latter, are conclusive 
evidence that the two formations are distinct in nature, also that the 
felsites are eruptive, and that their extrusion has occurred since that 
of the granite. 
That the felsites do not, by an insensible transition, pass into and 
become a part of the stratified group is shown by the markedly 
different characters of the two classes of rocks, as well as the 
distinctness of the boundary between them, and their different rela- 
tions to the granite. 
Chemical analysis, microscopical and petrographical relations, 
indicate that there are two kinds of felsitic rocks in this region. 
These have been provisionally called felsite and “ basic felsite.” 
The banding in the felsites is a fluidal structure, and not the 
result of sedimentation. This is shown by the fact that the banding 
occurs’in dikes of felsite, and is parallel to their walls. The micro- 
scopic structure of the felsites is proof of the same conclusion. 
The relations of the felsites to the breccia, and also the distinct 
junctions between two varieties of felsites, show that they are not 
