1880 .] 
113 
[Crosby. 
All the indications are plain that the slate was deposited in a 
bay having essentially the same outlines as Frenchman’s Bay, in 
other words, that Frenchman’s Bay is a very ancient sheet of 
water; and it certainly is a significant fact that these slates are not 
found on the main land outside of the immediate vicinity of the 
shores of the bay. 
The slate formation is traversed extensively by basic eruptives. 
These present the same general appearance throughout the basin, 
except as regards variations of 'texture depending on the sizes of 
the dikes, and differences due to unequal decomposition. They 
are conqiact to quite coarsely crystalline rocks, presenting a strong 
resemblance to the eruptive diorites associated with the surround- 
ing crystalline formations ; and they are probably identical with 
these in origin. 
I am informed by Dr. M. E. Wadsworth that the exotics cut- 
ting the slate, on Seward’s Island and probably at other points, 
are diabase. I have not studied these rocks microscopically, and 
hence, throughout this paper, when diorite is mentioned, diorite or 
diabase should be understood. 
The diorite cutting the slate is, on the average, somewhat finer 
grained than that penetrating the Huronian schists, which may 
indicate its eruption at a more recent period : and it forms far 
more regular, dike-like masses, which may be attributed in part to 
the wonderful regularity of the jointing of the slate as compared 
with the jointing of the intersected crystallines. The dikes vary 
in width from a few inches to several hundred feet, and, although 
usually cutting the strata, sometimes occur as intrusive beds. 
The stratification is not often disturbed by the presence of the 
dikes; but in many cases the slates are thoroughly baked and 
indurated in the vicinity of the contact, forming a semi-porcelainite 
or jasper, according as the slate is chiefly argillaceous or siliceous. 
At some points, notably along the Mount Desert shore south of 
Bar Harbor, the igneous action has been unusually intense, and 
the slates are literally torn to pieces by the intrusives. This is 
shown to the best advantage at Schooner Head. Nearly the 
entire promontory is composed of di'orite, traversed in some parts 
by exceedingly irregular and tortuous veins of feldspar and feld- 
spathic granite, with some veins of quartz, and enveloping 
PROCEEDINGS B. S. N. H. VOL. XXI. 8 OCTOBER, 1881 . 
