214 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The amount of material which is kept in constant movement by the 

 action of the sea within the outer line of ledges is very great, and is con- 

 stantly increased by the additional material derived from the breaking 

 up of the outer ledges by seas of unusual violence. The outer as 

 well as the inner ledges become disconnected ; passages between them 

 are opened with four fathoms of water, or even more. The outer 

 slope of the ledges is greater than the inner slope, the depth of water 

 increasing in some places rapidly from the ledges which are awash at 

 low water to six or seven fathoms, or even up to twelve off Castle 

 Harbor. From that depth, judging from the soundings on the hydro- 

 graphic charts, the slope is quite gradual, twelve fathoms usually being 

 found at a distance of nearly a mile from the outer ledges ; off the outer 

 side of the western reefs the slope is somewhat steeper, and off the north- 

 eastern face of the reefs from the Western Blue Cut to North Rock the 

 slope is slightly flatter. 



Two small disconnected banks exist to the southwest of the Ber- 

 mudas with a least depth, the one of twenty-one, the other of twenty- 

 four fathoms (Plate II.). A depth of over 1,000 fathoms has been 

 sounded between the main Bank and the Challenger Bank, while one 

 of 580 fathoms has been obtained in the channel between it and the 

 Argus Bank. The bottom in these is stated to be coral sand, and these 

 banks may, like the Bermuda Bank itself, be the summits of volcanic 

 peaks which have risen from a greatest depth of more than 2,000 fathoms 

 at a distance of about nine miles from the 100 fathom line. 



The Bermudas present quite a different physiognomy from that of the 

 Bahamas. The hills are more diversified in shape, many of them quite 

 conical, surrounding lowlands or sinks of considerable extent and of great 

 variety of shape. Along the northern shore of St. George Island, ex- 

 cept towards the east end, the hills are undermined as they reach the 

 water line, forming low cliffs in marked contrast to those of the south 

 shore, or on the waters of Harrington Sound and St. George Harbor and 

 the northern coast of Castle Harbor, where in certain places the water has 

 undermined the cliffs, and from them have dropped off huge masses leav- 

 ing vertical sides of considerable height, from fifty to seventy feet. On 

 the north shore only a few short stretches of the coast are occupied by 

 sand beaches. Shelly Beach is the only one of considerable length. On 

 the south shore we find sand dunes and many long stretches of broad 

 sand beaches (Plate XI.) which supply the material for the dunes that 

 are still in some places forcing their way inland over the ancient dunes, 

 as at Tuckerstown, Elbow Beach (Plate XII.), and Whale Beach. 



