224 J. W. SPEXCER SUBMARINE VALLEYS 



paper, the object of which is to record the facts given, with a brief state- 

 ment of the writer's explanation of these most interesting phenomena. 



Summary and Conclusions 



The coastal plains of the continent pass under the sea and extend for 

 20 miles off cape Hatteras, 70 to 80 miles off New Jersey and beyond, 

 again somewhat reduced in breadth, but widening out to 300 miles south- 

 east of Newfoundland, now forming the banks. Their gradient is almost 

 unappreciable to a depth of 200 to 250 feet, hi front of this margin is a 

 lower terrace, extending for a few miles to a depth of 400 to 450 feet, 

 bej^ond which the great continental slope begins. The terrace form is 

 best shown on the Newfoundland banks, while the outer slope of the 

 submerged plain is wanting off cape Hatteras. These plains are often 

 incised with channels to depths of 100 to 200 feet or more, suggesting 

 their more reccent elevation of 400 to 600 feet. Adjacent to the coast the 

 channels are apt to be filled b}^ delta deposits or by sands drifted by the 

 currents during the slight changes of level of the land, in the same way 

 that great sandbars, with lagoons behind them, have been formed along 

 the Atlantic coast. 



The Laurentian valley has a breadth of 30 to 60 miles, with a depth 

 of 1,200 to 1,500 feet below the submerged continental plain. The 

 Fundian valley and its tributaries have a somewhat less depth. The 

 drowned lands in this region have the same general ph3'sical character- 

 istics as the coastal plain farther south, except the irregularities due to 

 drift and to the less prevalent coastal sands, so that the Tertiary deposits 

 of the banks may be seen, traversed by river-like channels. 



Although the Laurentian and Fundian valleys are of greater age, they 

 were kept open or reopened in the late geological epoch when channels 

 were made in Tertiary beds. They were a feature when the land stood 

 about 2,000 feet above the present altitude and not much higher, else 

 the deeper outer valleys would have receded farther landward ; but the 

 margin of the continental shelf is everywhere incised in continuation of 

 the channels of existing rivers by coves, amphitheaters, or gulfs to depths 

 of 2,000 to 3,000 feet, where the submergence of the plains is from 250 

 to 400 feet or a little more. These features >re the same as those on 

 the border of the Mexican tableland. The submarine canyons widen 

 into valleys, more or less apparent, to the foot of the great continental 

 slope. So noticeable are they in contouring the soundings that they 

 force our attention to them, although the data is not as full as could be 

 desired. In descending the slope there are apparently lower gulfs or 



