TRANSACTIONS OF SECTION FE, 793 
other benthonic organisms which make up coral reefs and require a temperature 
of over 60° F. all the yearround. On the other hand, more than half of the surface 
of the ocean has a temperature which never falls below 60° F. at any time of the 
year. In these surface-~waters with a high temperature, the shells of pelagic 
Molluses, Foraminifera, Algze, and other planktonic organisms are secreted in great 
abundance, and fall to the bottom after death. 
It thus happens that, at the present time, over nearly the whole floor of the 
ocean we have mingled in the deposits the remains of organisms which had lived 
under widely different physical conditions, since the remains of organisms which 
lived in tropical sunlight, and in water at a temperature above 80° F., all their 
lives, now lie buried in the same deposit on the sea-floor together with the remains 
of other organisms which lived all their lives in darkness and at a temperature 
near to the freezing-point of fresh water. 
Marine Deposits on the Ocean-floor. 
The marine deposits now forming over the floor of the ocean present many 
interesting peculiarities according to their geographical and bathymetrical position. 
On the continental shelf, within the 100-fathom line, sands and gravels predomi- 
nate, while on the continental slopes beyond the 100-fathom line, Blue Muds, Green 
Muds, and Red Muds, together with Volcanic Muds and Coral Muds, prevail, the 
two latter kinds of deposits being, however, more characteristic of the shallow 
water around oceanic islands. The composition of all these Terrigenous Deposits 
depends on the structure of the adjoining land. Around continental shores, except 
where coral reefs, limestones, and volcanic rocks are present, the materials consist 
principally of fragments and minerals derived from the disintegration of the ancient 
rocks of the continents, the most characteristic and abundant mineral species being 
quartz. River detritus extends in many instances far from the land, while off high 
and bold coasts, where no large rivers enter the sea, pelagic conditions may be 
found in somewhat close proximity to the shore-line. It is in these latter positions 
that Green Muds containing much glauconite, and other deposits containing many 
phosphatic nodules, have for the most part been found; as, for instance, off the 
eastern coast of the United States, off the Cape of Good Hope, and off the eastern 
coasts of Australia and Japan. The presence of glauconitic grains and phosphatic 
nodules in the deposit at these places appears to be very intimately associated with 
a great annual range of temperature in the surface and shallow waters, and the 
consequent destruction of myriads of marine animals. As an example of this 
phenomenon may be mentioned the destruction of the tile-fish in the spring of 1882 
off the eastern coast of North America, when a layer six feet in thickness of dead 
fish and other marine animals was believed to cover the ocean-floor for many 
square miles, 
In all the Terrigenous Deposits the evidences of the mechanical action of tides, of 
currents, and of a great variety of physical conditions, may almost everywhere be 
detected, and it is possible to recognise in these deposits an accumulation of mate- 
rials analogous to many of the marine stratified rocks of the continents, such as 
sandstones, quartzites, shales, marls, greensands, chalks, limestones, conglomerates, 
and volcanic grits. 
With increasing depth and distance from the continents the deposits gradually 
lose their terrigenous character, the particles derived directly from the emerged 
land decrease in size and in number, the evidences of mechanical action disappear, 
and the deposits pass slowly into what have been called Pelagic Deposits at an 
average distance of about 200 miles from continental coast-lines. The materials 
composing Pelagic Deposits are not directly derived from the disintegration of the 
continents and other land-surfaces. They are largely made up of the shells and 
skeletons of marine organisms secreted in the surface waters of the ocean, consist- 
_ ing either of carbonate of lime, such as pelagic Molluscs, pelagic Foraminifera, and 
pelagic Algze, or of silica, such as Diatoms and Radiolarians. The inorganic con- 
stituents of the Pelagic Deposits are for the most part derived from the attrition of 
floating pumice, from the disintegration of water-logged pumice, from showers of 
