Annual Meeting.] 
12 
[May 7, 
immense floods of lava were also poured out on the surface, conceal- 
ing the sedimentary rocks. In breaking through the quartzite and 
slate, the diorite has often followed the planes of stratification in 
those rocks, and thus appears in many cases to be regularly inter- 
stratified with them ; and this bedded appearance of the diorite is 
greatly increased by the very perfect flow-structure which was de- 
veloped in large masses of it, as well as by the distinctly schistose 
or foliated structure sometimes resulting from the subsequent action 
of both mechanical and chemical forces. We are thus able to ex- 
plain the fact that the diorite has been frequently mistaken for a 
stratified or sedimentary rock. 
The eruption of the diorite was probably followed by a prolonged 
period of quiet erosion, which was finally terminated by the advent 
of a second period of intense and long continued igneous action, 
during which only acid rocks, the granites and felsites, were formed. 
All over this region the granite has broken through the Primordial 
strata and the diorite in the most irregular and intimate manner, 
sometimes developing a flow-structure similar to that in the diorite, 
causing the granite to be mistaken for gneiss, and very often follow- 
ing the flow-structure or foliation of the diorite in thin layers or 
veins, thus producing an imperfect blending of the two rocks and 
heightening the stratified appearance of each. Over large areas the 
diorite and granite are so completely mixed in these various ways 
that it is impossible to represent them separately on the map. The 
granite embraces many different varieties and some of these are 
clearly older than others ; although all, so far as known, are newer 
than the diorite. Syenite is well developed in the vicinity of Salem 
Harbor and to some extent in other localities. It is newer than most 
if not all of the granite, forming dikes in the latter ; but it is very 
similar to the granite in its relations to the diorite. 
The felsite is less widely distributed than the granite, occurring 
mainly in the immediate vicinity of the Boston Basin. It not only 
occurs as dikes in the Primordial slate and quartzite and the diorite, 
but also in the granite. It is unnecessary, however, to suppose 
that it is widely separated from the granite in time ; but it seems 
best to regard it as in general the most superficial and newest part 
of the granitic eruptions. That the felsite was formed chiefly as 
surface flows of acid lava, resembling, when new, obsidian among re- 
cent lavas, is clearly indicated by the banding or flow-structure, the 
brecciation, and other structural features of the numerous varieties. 
