TERRACES AND RIVER VALLEYS OP WESTERN EUROPE. 271 



channels down to depths of several thousand feet below the 

 present level, it would probably not have occurred to me to 

 ascertain whether similar physical features characterise the 

 bed of the ocean along its eastern margin.* 



II. Mode of determining sub-oceanic physical features. 

 — Isobathic lines (or lines of equal depth), drawn on the 

 charts by the aid of soundings, offer a reliable means for 

 determining the physical features of submerged areas in the 

 same way that contours traced by means of levelling, serve 

 for representing those of the land. When the soundings are 

 sufficiently numerous, as is tlie case on the Admiralty charts off 

 the British coasts, the isobathic lines may be drawn at short 

 intervals of depth, and the form of the sea-bed may be very 

 accurately drawn in section ; but off the coasts of Spain and 

 Portugal, as in the case of Vigo Bay, additional scmndings 

 are much to be desired in order to enable us to delineate 

 with sufficient accuracy the contours of the oceanic bed. 

 It may, nevertheless, be affirmed that those shown on the 

 Admiralty charts are quite sufficient to enable us to trace 

 oat the maiu features of the ocean floor; and, in some 

 instances when the coast is approached, with all needful 

 minuteness of detail. For the purpose of this investigation 

 I have found the isobaths of 100, 200, 500, 750, 1,000, and 

 1,500 fathoms, generally sufficient except in a few special 

 ■cases where additional contours have been drawn. Beyond 

 the 1,500-fathom contour, the gently sloping floor of the 

 abyssal ocean, formed mainly of " Globigerina ooze," spreads 

 away westward. On the British Admiralty charts, the 

 nature of the sea bottom is frequently indicated along with 

 the depth of each sounding. 



III. Extent of the region embraced in this paper. — 

 Having already dealt with the tract lying off the British 

 Isles extending from the platform of Rockall round by that 

 of the west coast of Scotland, England and Ireland as far 

 south as the English Channel,! I propose to extend our 



* Spencer, " Reconstruction of the Antillpean Continent," Bull. Geol. 

 jSoc. Amer., vol. vi ; Warren Upham, "Cause of the Glacial Epoch," 

 Trans. Victoria Inst., vol. xxix, p. 121. Several American geologists, 

 including Dr. Walcott, have given their adhesion to Professor Spencer's 

 views. 



t " Further investigations regarding the Submerged Terraces and River 

 Valleys bordering the British Isles," Trans. Victoria Inst., vol. xxx, 

 p. 305. 



