470 HENRY S. WASHINGTON 
abundance of the albite molecule, lack of lime-soda feldspars, 
and presence of nepheline, aegirite, and blue glaucophane- 
riebeckite or brown barkevikite. These rocks show high Na, O: 
KO: and Na, O-)K7 © :Si@; ratios and low HeOr her Ore 
Related to this series are the dikes of the sdlvsbergite- 
tinguaite series, and of paisanite, also possibly the camptonitic 
dikes. These show chemical and mineralogical characters 
analogous to those of the foyaitic series, but vary far more in 
composition. i | 
These four series, which are very well defined, include nearly 
all the rocks examined. Among the exceptions the horn- 
blende-gabbro occupies, as we have seen, a position intermediate 
between the diorites and essexites, and may be reasonably 
regarded as a transitional and connecting form. The flow rocks 
are abnormal. In certain respects they seem to be allied with 
the granito-dioritic rocks, while other characters suggest affinities 
with the foyaites. The question is a difficult one to decide. 
The orbicular syenites, which are present in very small amount, 
are almost certainly related to the granito-dioritic series, though 
lack of an analysis leaves the question uncertain. By its mineral- 
ogical characters and by its ratios the Quincy granite belongs to 
a foyaitic series, forming the most acid member of it, and cor- 
responding to the paisanites among the dike rocks. As it belongs 
to the Blue Hills Complex, quite outside our region, it will not 
be discussed further. 
Relations of the various types—As a preliminary to the deter- 
mination of the genetic connection of the various rocks, it will 
be well to obtain some idea of the relative amounts of the 
different types represented. For obvious reasons it is, of course, 
not possible to do this with certainty or accuracy. If we assume 
that the areas shown on the geological map are dependent on 
the relative volumes (which may or may not be the case) we 
shall get an estimate, which, though very far from being accurate 
or wholly satisfactory, may be considered provisionally to - 
express the relation in a general way, and which will probably 
be sufficient for our present purpose. 
