Crosby.] 
110 
[December 15, 
between the structure and distribution of the crystalline rocks and 
the contours of the bay, which is very clearly the product of ero- 
sion and not of subsidence. 
The mica schist is an obscurely crystalline, slaty-looking rock, 
always distinctly bedded in thin layers, rich in quartz, which is 
usually very compact and mixed with more or less argillaceous 
material, and poor in mica, which is sometimes hydrous, never 
occurs in large scales or spangles, and is in most cases a very 
inconspicuous constituent of the rock. Occasionally a little feld- 
spar is observable in the rock, making a poor sort of gneiss ; but 
more often it passes, through the absence of the mica, into quartz- 
ite or a siliceous argillite. The argillaceous beds, especially, are 
dark colored; and in these, too, the stratification is often very 
obscure. They are sometimes of unusual density and evidently 
contain hornblende as an important constituent, making an argilla- 
ceous hornblende schist ; while in other cases true stratified diorites 
are observed, composed of feldspar and hornblende. 
The stratified group must be classed as one of the metalliferous 
formations of this region, since it encloses the silver-bearing vein 
at Sullivan Falls. It is also undoubtedly identical with the cuprif- 
erous series of Blue Hill, in which the copper (clialcopyrite) is, 
however, almost entirely indigenous. In fact, the rocks in question 
belong to an extensive formation bordering the coast at intervals 
from the Penobscot to New Brunswick, which has been referred 
to the Huronian System by Dr. Hunt and Prof. Hitchcock. This 
series evidently dates from a very early period, and I see no reason 
to question its Eozoic age. 
Both the eruptive diorite and the granite agree in their general 
distribution with the stratified group. In the vicinity of French- 
man’s Bay, however, the diorite is found chiefly in East Sullivan 
and Gouldsborough ; and in the latter town it is associated with 
veins of chalcopyrite and argentiferous galenite. The diorite is 
usually very dark, being rich in hornblende, and ranges in texture 
from quite coarsely crystalline to aphanitic. The more compact 
varieties, especially, are often very intimately associated with the 
stratified diorite already mentioned ; and there are indications that 
here, as in eastern Massachusetts, a gradual passage exists between 
these two rocks. 
