Formation of Sub-oceanic Strata. 129 



marine denudation, of organic remains derived from plants and 

 animals, and of substances held in solution, such as salts, gases, 

 &c. In accordance with the above-mentioned conditions, we 

 may expect the solid particles to form deposits varying in 

 quantity and quality in proportion to the distance to which they 

 have been carried, and to the greater or lesser velocity of the 

 currents which occupy the area in which they have been deposited. 

 This conclusion is borne out by facts which have come to 

 light in the course of the recent researches into the nature and 

 composition of the deposits found at the bottom of the sea. 



The samples brought up from the bottom in the tube of the 

 sounding apparatus reveal a marked difference between deposits 

 formed near the land and deposits accumulated in the more central 

 parts of an oceanic basin. This difference is sufficiently great 

 to render it possible — as soon as we shall possess a complete 

 analysis of the specimens already collected — to decide whether 

 a certain sample of the sea-bottom, the origin of which may be 

 doubtful, belongs to a stratum deposited in a deep sea or in a 

 shallow sea, near the margin or near the centre of an oceanic 

 basin. 



It is evident that a large proportion of the detritus derived 

 from sub-aerial and submarine denudation, including all the 

 heavier and at the same time more voluminous particles, will be 

 deposited within a short distance from the margin ; that the com- 

 position of this marginal deposit will depend upon, and vary with, 

 the materials which make 4ip the surface-strata of the adjoining 

 land ; and that the distance to which it extends from the shore 

 will be influenced by the presence or absence of shore-currents, 

 or of rivers emptying themselves into the sea. Thus the breadth 

 of the marginal deposits may amount to several hundred miles at 

 the mouth of great rivers, such as the Amazon, the Rio de la Plata, 

 the Mississippi, &c, while it may be reduced to a few miles in places 



where the shore is swept by powerful currents. Hence, under 

 1 2 



