10 THE GRANITES OP MAINE. 
is almost completely surrounded by a border zone, from one-fourth 
mile to H miles in width, in which the rocks are largely diorite and 
gabbro with small amounts of igneous gneiss and fine-grained granite. 
Some hint as to the cause of this contrast in the character of 
granite borders in different regions is obtained from a study of the 
rocks in the southwestern part of the State, especially in Sagadahoc, 
Cumberland, and Oxford counties and the southern part of Kennebec 
County. Here, as indicated on the map (PL I) , there are considerable 
areas from which large continuous masses of normal granite are 
absent, but in which the prevailing sedimentary schists have been 
intruded in the most intimate manner by dikelike or irregular masses 
of pegmatite and fine-grained granite, and in many places have been 
given a gneissic texture by the lit-par-lit injection of granitic mate- 
rial. The intruded and injected areas pass gradually into the larger 
areas of nearly pure normal-textured granite shown on the map. To 
explain such intimate injection and intrusion in areas far removed 
from any outcropping masses of pure normal granite it seems neces- 
sary to assume that a granite mass underlies these 4 rocks at no great 
distance below the present surface and that such injected areas con- 
stitute in reality portions of the " roof " of great granite batholiths. 
It seems almost certain that the escape of gases and water vapor 
and the differentiation of basic rocks from the granitic magma would 
proceed much more rapidly from the upper surface of a buried 
magma than from its sides. It is to be expected, therefore, thai por- 
tions of the "roof" of such granite masses should be particularly 
characterized by the abundance of pegmatites, diorite, gabbro, etc.. 
and by notable contact metamorphism of the sedimentary rocks 
through which these forced their way. The sharpness of other 
granite contacts is readily explained by supposing that they represent 
the side contacts of the granite batholiths, where the gases and water 
vapors escaped from the magma laterally in much less volume and 
where the accompanying metamorphic effects were very much less 
than at the upper surface. 
The geologic history of the great granite intrusions of Maine may 
be summarized, therefore, as follows : 
All of the granite masses now exposed solidified below the surface 
as it existed at the time they were intruded. The depth at which 
they solidified varied in different places. Erosion gradually re- 
moved the rocks covering some of the masses and has in some places 
even revealed their deeper portions, so as to show the sharp lateral 
contacts. In other places all or a part of the " roof " of the granite 
masses still remains. The present land surface, therefore, truncates 
the various granite batholiths at different horizons. It is highly 
probable that a further erosion of 500 to 1,000 feet would expose 
much larger areas of granite than now appear. 
