1880 .] 
Ill 
[Crosby. 
The areas covered continuously by the diorite are quite exten- 
sive in some cases, including many acres or even square miles, 
though with isolated patches of the stratified group here and there. 
The outlines of this rock are very irregular for the most part, and 
it rarely forms well defined dikes. It intersects all the members 
of the stratified group and also the granite. 
The most characteristic variety of granite is a firm, coarsely 
crystalline aggregate of quartz, red or pinkish orthoclase, and 
hornblende. The orthoclase is the predominant constituent and 
determines the color of the rock, while the hornblende is rarely 
conspicuous. Mount Desert is mainly formed of this coarse, red, 
hornblendic granite ; and Green Mountain, especially, is a magnif- 
icent development of it. It is also the prevailing, and almost the 
only rock in all the southern and western part of Gouldsborough. 
Although typically coarse, yet the texture of the granite in 
many places is decidedly fine, and even, macroscojfically, almost 
felsitic. These finer grained varieties are most abundant in Sulli- 
van ; and in this direction, especially, the orthoclase is frequently 
white. A white or gray hornblendic granite of medium texture 
is quarried on a large scale at Sullivan Falls. Mica rarely occurs 
in these granites, and is never abundant, so far as I have observed. 
Coming now nearer to my proper subject, the geology of French- 
man’s Bay, I would remark, in the first place, that, while the strat- 
ified crystalline group above described appears on the shore at 
numerous points in the north part of the bay, the only stratified 
rocks exposed on the islands, so far as I have observed, and on the 
east side of Mount Desert, are entirely uncrystalline and belong 
to a distinct and much later age. 
This is essentially a slate formation, the prevailing rock being a 
compact, well jointed argillite or clay slate of black, drab and 
purple tints. Although usually firm and strong, the slate is some- 
times thin bedded and fissile, a true shale. The stratification is 
always perfectly distinct. The slate is often evidently siliceous, 
and passages into sandstone are common, while the formation 
includes some true quartzite. On one of the Porcupine Islands a 
stratum of slate about thirty feet thick weathers almost snow 
white, having, at a little distance, the appearance of a bed of lime- 
stone. It evidently contains some undecomposed feldspar. 
