CAPE COD GEOLOGY 



229 



THE BLACK-IRON SAND OR MAGNETITE 



The east shore of Block Island, along the bathing beach, abounds in mag- 

 netic iron sand, which is blown up into low dunes, from which, in years past, 

 the market was supplied with blotting sand in the days of quill pens. (Plate 32, 

 fig. 1.) Many years ago a large horseshoe shaped magnetic separator was devised 

 and installed in a factory on the beach. The remains of this instrument, in- 

 cluding the huge magnets, were seen as late as 1916. (See Plate 32, fig. 2.) 



In the early 90's Edison 1 investigated the black sand of Long Island by 

 boring holes along the beach and inland. He found that along the beach in one 

 year there was an aggregate thickness of 12 to 15 inches of black sand for 8 or 

 10 miles ; yet in the next year hardly any could be found. He therefore abandoned 

 the attempt to treat the deposits commercially. 



The black magnetic iron sand of Block Island is washed out of the glacial 

 drift along the shore by the sea. The drift on the island carries much waste 

 derived from beds of magnetiferous schist of Carboniferous age in the Narragan- 

 sett Bay region. These beds are probably the chief source of the magnetite, 

 but minute crystals of magnetite are abundant in many igneous and crystalline 

 rocks of southern New England, and Shaler found that traces of magnetite 

 are very generally distributed in the glacial sand in this region. 



GREAT SALT POND 



The largest body of enclosed water on the island is Great Salt Pond, which 

 has an area of about 1,000 acres. It occupies a great depression between the 

 northern and southern parts of the island and in places reaches depths of 52 

 feet below mean sea level. The bottoms of Trims and Harbor ponds, which 

 lie southeast of Great Salt Pond, also lie below sea-level. Great Salt Pond is 

 separated from the sea on the west by a cordon of beach sand, which ties in two 

 small islets of glacial sand. Between these islands a navigable canal has been 

 dredged to a minimum depth of 25 feet. An earlier canal dug north of these 

 islets is now filled with sand. Crescent Beach, on the east side of Great Salt 

 Pond, forms a barrier extending from the Harbor to the southern part of Corn 

 Neck, but it includes islets of glacial material, from which much coarse debris 

 is eroded. Between the beach and Great Salt Pond a small islet, which has cliffs 

 on its west side, supports a small barrier beach that partly separates Trims 

 Pond from Great Salt Pond. 



Great Salt Pond, by reason of a channel cut through the western barrier 



1 Edison, Thomas E., Letter to J. B. Woodworth dated Nov. 22, 1893. 



