44 The American Geologist. Jul - V - 19U5 
section shows both the normal granite below the fine granite 
and the quartz porphyry above it. In other words, we 
have no data for a direct determination of the thickness, 
«ave that it must exceed the hight of the highest hill com- 
posed wholly of the fine granite, or say 75 feet. It would 
be readily deducible from the surface breadth if the dip 
-were known. Assuming the dip to be low and inversely 
proportional to the surface breadth, gives a maximum thick- 
ness of a few hundred feet at the most; and 100 to 200 feet 
may, perhaps, be accepted as a conservative estimate, con- 
firming the conclusions reached in the study of the Blue 
Hills complex. 
The finer granite of the contact zone is, in a fair sense, 
a bed of passage, since it grades downward into the nor- 
mal granite and upward into the quartz porphyry ; and, 
normally, its original contacts are nowhere sharply de- 
fined, but distinctly blending. It may be noted, however, 
that, as in the Blue Hills complex, the contact with the 
normal granite, though blending, is rather abrupt, the com- 
plete transition from the one rock to the other being ac- 
passage upward into the quartz porphyry is usually more 
or, possibly, in extreme cases, a single foot. Although the 
complished in some exposures in the breadth of a few feet 
.sedimentary rocks. 
gradual, all observers must recognize that the fine granite, 
•so far from being all gradation, is chiefly remarkable for the 
uniformity of texture throughout almost its entire thick- 
ness. In fact, it rivals the normal granite in this respect. 
Locally, and especially near the quartz porphyry, it may 
pass into a true microgranite ; but it is in general a macro- 
granite of very homogeneous aspect.* That the fine granite 
is older than the normal granite and younger than the 
quartz porphyry, and that these three distinct but blend- 
ing sedentary zones of the batholite exhibit the structural 
relations which this sequence requires, will probably not be 
questioned by those familiar with the field evidence. 
* The explanations of the homogeneity of the fine granite and its 
abrupt yet blending passage into the normal granite suggested in Part 
III. of the Boston Basin Geology (Occas. Papers, Boston Soc. Nat. 
History, iv, 354 et seq.) are still 1 egarded as valid, and as applicable 
in this new field. 
