202 Proceediwis Port. Soc. Nat. Hint. 



this has rocky shores and no development of swamps or bogs 

 about its margins. 



The third type of pond is the brackish barrier beach pond, 

 formed by the cutting off of some little indentation of the 

 sea by the deposition of beach sands and gTavels or of wave- 

 worn rocks. They occur along the more shallow shores on 

 the outer islands where opportunity is afforded for the neces- 

 sary wave action. As in the case of the morainal ponds 

 these show all stages of transition to swamps and uplands. 



Geology 



The rocks' which underlie the eastern Penobscot Bay re- 

 gion are for the most part highly silicious and consequently 

 acidic in nature.' Granite is exceedingly abundant, occupy- 

 ing fully three-quarters of the whole area, especially at the 

 higher elevations. Most of the smaller islands are entirely 

 made up of this rock, while on the larger, such as Swans 

 Island and Deer Isle, there are areas many miles in extent. 

 The second most abundant formation, particularly on the 

 mainland and Deer Isle, is a sedimentary schist, "rich in 

 micaceous minerals and quartz." On some of the outer 

 islands there are small amounts of diorite, diabase, and 

 gabbro. These vary in composition from a '^quartz diorite 

 only slightly different from granite to a much more basic 

 rock." On the western side of Deer Isle and on Isle au 

 Haut are found various volcanics, occurring as flows, breccias, 

 pyroclastics, and other surface forms. Since these rocks, 



1. statements regarding the rocks of this region are taken from 



U. S. Geol. Survey, Penobscot Bay Folio 149. 



2. If the chemical nature is considered, rocks are grouped as acidic 



(over 65% silica), basic (less than 55% silica), and intermedi- 

 ate (from 55%-65% silica). See Merrill, Rocks, Rock 

 Weathering and Soils. New York. 1913; p. 59. 



