GEOLOGICAL AND MINERALOGICAL NOTES. 69 



During the past summer I have made soundings in Sa- 

 lem and Marblehead harbors for the purpose of comparing 

 the depths of the water over certain rocks with those given 

 in the report and on the chart prepared by Dr. Nathaniel 

 Bowditch in 1804 and 1805. In his report Dr. Bowditch 

 states that the summit of Bodin's rock was seven feet be- 

 low low water on the füll and change of the moon, taken 

 from easily recognized compass points on the main land 

 and islands in the harbor. Soundings taken with an iron 

 rod on this spot, the first July 17, 1894, low water 6 

 a. m., füll moon, gave 9 feet of water; again taken 

 August 1, 1894, new moon, low water 5.28 p. m., gave 

 8J feet of water at the same spot. These soundings were 

 made with care and are reasonably correct and, in this 

 case, off er evidence of a subsidence in the past ninety 

 years at least of one and one-half feet at this point. 



Dr. Bowditch's report gives 5 feet of water, at mean 

 low water, on the summit of Privy ledge, 300 yards 

 outside Orne's Island. August 2, 1894, new moon, low 

 water 5.28 a. m., there was 7 feet of water at this point, 

 indicating a subsidence of 2 feet. There is, however, in 

 all probability a greater amount of erosion at this place 

 than on Bodin's rock in the harbor. Dr. Bowditch reported 

 6 feet of water on the shoalest portion of Abbot's rock, 

 Salem harbor, while on August 30, 1894, new moon, low 

 water, I found 8 feet. Taken at low water, August 31, 

 1894, Archer's rock had 8 feet of water; September 1, 

 1894, Bowditch's ledge had 7J feet, and September 2, 

 1894, Cut-throat ledge had 6 feet of water. In Dr. Bow- 

 ditch's report 6-7 feet of water is given for Archer's 

 rock which is 1 foot less than I lind it. He gives for 

 Bowditch's ledge 5-6 feet of water where niy soundings 

 gave 7 i feet. On Cut-throat ledge Bowditch gives 4 feet 

 of water, while I lound 6 feet at extremely low water. 



