248 THE ATLANTIC. [chap. v. 



at the bottom of a deep western trough ; the water then shoaled 

 rapidly up to the West Indian Islands. 



On our next section from St. Thomas to Bermudas we 

 sounded a little to the north of the Virgin Islands in 3875 

 fathoms, the greatest depth known in the Atlantic, and our 

 whole course lay through a depression upward of 2500 fathoms 

 deep, showing that the western trough extended considerably 

 to tlie northward. This western valley was again traversed be- 

 tween Bermudas and the Acores, the water shallowing at a dis- 

 tance from those islands, thus showing that tliey formed the 

 culminating points of a plateau of considerable extent. Be- 

 tween the Agores and Madeira we recrossed the eastern valley, 

 and our course from Madeira to the Cape Yerde Islands, and 

 southward to a station in lat. 5° 48' N., long. 14° 20' W., lay 

 within it, near its eastern border. We then crossed the valley, 

 and in lat. 1° 22' N., long. 26° 36' W., we sounded in 1500 fath- 

 oms near the centre of the middle ridge, and, altering our course 

 to the south-westward, we crossed obliquely a western depres- 

 sion, with a maximum depth of about 2500 fatlioms, between 

 St. Paul's Bocks and Cape St. Roque. From Bahia we crossed 

 a western depression with a maximum depth of 3000 fathoms, 

 and came upon 1900 fathoms on the central rise, a few degrees 

 to the westward of Tristan d'Acunha. An eastern depression 

 with an average depth of 2500 fathoms extended for the greater 

 part of the distance between Tristan d'Acunha and the Cape 

 of Good Hope. 



On our return voyage, in 1876, we crossed the western basin 

 of the South Atlantic about the parallel of 33° S. We then 

 ran northward on the top of the rise in the meridian of Tristan 

 d'Acunha and Ascension as far as the equator, and the greater 

 part of the remainder of our course lay nearly in the axis of 

 the eastern depression. 



Combining our own observations with reliable data which 

 have been previously or subsequently acquired, we find that 



