Rev. J. F. Blake — Form of Sedimentary Deposits. 17 



the depth to which they reach, which, except in the case of ocean 

 currents, is said to be not greater than 100 fathoms. Down to this 

 depth, therefore, the deposits from the suspended material carried 

 by rivers are liable to be supplemented by the contributions made 

 by the waves on the shore, and the contour-line of 100 fathoms will 

 thus be accentuated/ This is the ' mud-line ' of Dr. Murray, 

 or the most common limit of terrigenous deposits to which the 

 lightest particles — those of organic origin — are carried and form 

 a feeding-ground for fishes, etc. 



From this investigation it appears (1) that when a deposit 

 consists of pretty uniform material of fine grain it is likely to end 

 at the part most remote from its source in a steep slope seawards, 

 and that its thickness there is unlimited except by the depth 

 of the sea on which it is formed ; (2) that at a greater or less 

 distance from every shore there is likely to be found a narrow zone 

 across which there is sudden fall in the sea-bottom, in which the 

 contour-lines will be crowded together, but from which they may 

 diverge locally opposite the mouth of any muddy river, a higher 

 one towards it and a lower one away from it, and this zone is 

 liable to be specially narrowed and thus accentuated when it 

 occurs at a depth of about 100 fathoms ; and (3) that both these 

 results are due to the wedge-shaped form of the original sedimentary 

 deposits in the sea. 



Before proceeding to compare these results with nature, some 

 account must be given of the relations of limestones to the deposits 

 already dealt with. Limestone for the most part is introduced 

 into the sea in the form of calcareous salts in solution ; it does not 

 usually take a sedimentary form until it has passed through some 

 organism, and will therefore accumulate most where organisms 

 such as precipitate the salts are most abundant. For such organisms 

 to abound it is certainly necessary that there should be (1) abundance 

 of calcareous salts supplied, (2) abundance of oxygen to induce 

 activity, and probably also (3) freedom from mud. For the first 

 condition, nearness to the shore, the source of such salts, is necessary ; 

 for the second, nearness to the air, whence the oxygen must be 

 obtained ; but for the third, or least certainly necessary one, remote- 

 ness from the sources of terrigenous deposits. 



Since the discovery of pure Globigerina ooze at abysmal depths, 

 it has been assumed that the third condition outweighs the other 

 two and that limestone is an indication of deep water. Any such 

 assumption is inadmissible, if for no other reason than this, that the 

 Grlobigerinas which live on the surface and sink to the bottom when 

 dead are protected from all detrition ; they are, therefore, extremely 

 abundant in the ooze ; but in the purest chalk the greater bulk is 

 detrital, and must be washed away before the Foraminifera can even 

 be looked for. Globigerina ooze, therefore, throws no light on the 

 relation of limestone to depth. We must start again ab initio. 

 There is not at present sufficient information available to enable us 



^ See N. M. Fenneman, Journ. Geol. Chicago, vol. x, pp. 1-32. 



DECADE IV. VOL. X. NO. I. 2 



