EEPOET ON THE DEEP-SEA DEPOSITS. 
189 
h. The Different Types of Deep-Sea Deposits. 
I. Pelagic Deposits. 
We have just indicated that the deposits of the deep sea may be divided into two 
great groups distinguished by the terms “ Pelagic ” and “ Terrigenous.” Pelagic deposits 
are situated at a considerable distance from land, and for the most part in the greatest 
depths of the ocean. It is only in exceptional circumstances that sandy or other particles 
immediately derived from the land make up any considerable portion of these deposits. 
The most characteristic minerals are derived from volcanic eruptions, floating pumice, 
or are of secondary origin formed in situ. The remains of pelagic organisms that have 
fallen from the surface form the principal part of many of these deposits, as indicated 
by the names : Pteropod, Globigerina, Diatom, and Radiolarian Oozes. ‘ In some of the 
deeper regions of the ocean these organic oozes are replaced by Eed Clays, formed for 
the most part by the disintegration of rocks and minerals in situ. 
Generally speaking, the physical conditions in the areas occupied by the pelagic 
deposits are very uniform ; the temperature is near the freezing-point of fresh water, 
and the range never exceeds 7° F., being constant throughout the year at any one 
locality. Sunlight and vegetable life are absent, and, although animals belonging to all 
the principal groups are present, there is no great wealth either in the number of indi- 
viduals or of the species, though many of the latter may present archaic characters. There 
are but few indications of change of any kind, and the rate of accumulation of some of 
these pelagic deposits must be exceedingly slow, so that we apparently And the remains 
of Tertiary species lying on the sea-floor alongside those of species inhabiting our 
present seas. With some doubtful exceptions, it has been impossible to recognise in the 
rocks of the continents formations identical with these pelagic deposits.^ 
Before commencing the description of the different types of Deep-Sea Deposits, it may 
be well to repeat that while it is easy to distinguish one kind of Deep-Sea Deposit from 
another when dealing with typical samples, this becomes less and less easy when, with a 
change of conditions, a deposit gradually changes its characters and slowly assumes those 
of another. In this way it happens that there is at many points in the ocean a gradual 
transition from the one t 5 rpe of deposit into another, and generally it may be said that 
there is no sharp and distinct line limiting the areas occupied by the various kinds of 
deposits either in depth or geographical extension, as might be supposed from an 
examination of our map representing the distribution by means of colours. 
We commence the consideration of pelagic deposits, taking first the most characteristic 
type, — the clays formed in the greatest depths and greatest distance from dry land. 
^ J. B. Harrison and A. J. Jukes-Browne, The Geology of Barbados (published by authority of the Barbadian Legis- 
lature), 1890 ; H. A. Nicolson, Trans. Edin. Geol. Soc., vol. vi. p. 56, 1890 ; G. J. Hinde, Ann. and Mag. Nat. Hist., 
ser. yi. vol. vi. p. 45, 1890. 
