GENERAL CONCLUSIONS. 255 
It has here been shown that acidic and basic dikes (syenite-porphyry 
and diabase) of pre-Potsdam age occur in the Adirondack region. Similar 
diabases, both as dikes and effusives, range widely to the east and west. 
The syenitic rocks are rarer, though also with a wide range. Taken to- 
‘gether, they present a great similarity to the Keweenawan eruptives of 
the upper lake region, nor can they depart widely in age from them. 
Apparently the one region was depressed while the other was above sea- 
level, so that the clastics are lacking in the Adirondacks, but in both 
cases there is a similar relationship to an unconformable sandstone above 
and to a mass of gabbroic and granitic eruptives below. 
Three distinct periods of igneous activity have been differentiated in 
the Adirondack region, leaving entirely out of the question the origin of 
the doubtful gneisses. The latest occurred probably in Silurian times 
and gave rise to the bostonites and basic dikes along lake Champlain. 
The porphyries and diabases preceded these in pre-Potsdam times. Still 
earlier are the gabbros and granites, whose precise relations to one an- 
other have not been determined, but which are both younger than all, 
or at least nearly all, of the gneisses of the region. Some of the gabbros 
were later than others, but there is no evidence of any special interval 
between them. In each of these three instances there is an association 
of basic and acidic rocks, and in each a close agreement in chemical 
composition, accompanied, however, by mineralogical and structural dif- 
ferences which are constant throughout the district. 
Eyidence is constantly accumulating to show that three similar periods 
of igneous activity were characteristic of the entire shoreline of the an- 
cient Canadian and Appalachian protaxes. While much more evidence 
is requisite, interesting correlations are suggested, and are at least worthy 
of being borne in mind as working hypotheses. 
XXXVIIIT—Bor1, Grou, Soc. Am., Vou. 9, 1897 
