DEEP-SEA FORMATIONS. 151 
The absence of typical cretaceous belemnites, baculites, and 
ammonites does not in the least prove that a process similar 
to that which deposited the chalk of the cretaceous period may 
not be going on at the present day on the floor of the Atlantic. 
But owing to the variable nature of the bottom, it does not 
follow by any means that the same process is going on syn- 
chronously all over the Atlantic bed. Deep-sea muds, such as 
we find on the steep slopes like those of Hatteras, the Wind- 
ward Passage, and the continental slopes of the Gulf of Mexico 
and the Caribbean, can be compared to fossiliferous shales. 
How far the presence of deep-water types can be traced to 
paleozoie times is very uncertain. АП the evidence thus far 
tends to show that the deep-sea fauna originated at the close of 
the palæozoic times, for many of the genera which we have been 
led to consider as deep-sea types in the mesozoie period lived 
in comparatively shallow water. The Lingula of to-day is a 
shallow-water dweller; it may have been one then. The pre- 
sence of pteropods and thin-shelled cystideans, and of many 
blind trilobites, would seem, however, to indicate the existence 
of invertebrates at considerable depths along the continental 
slopes even in these early days. Deep-sea faune, like all marine 
faunse, are essentially dependent upon the nature of the bottom; 
in former times, globigerinz and siliceous sponges must have 
flourished on calcareous ooze; mollusca and annelids character- 
ized muddy bottoms, while polyps, gorgoniz, and corals throve 
on rocky ground. It is to be noticed that those of the older 
formations which were probably deposited in deep water, 
always appear to be of far greater thickness than the so-called 
littoral beds. This would show that there was in past times 
as great a variety of conditions in which marine animals lived 
as we find at the present day, and that the limits of tempera- 
ture were fully as great; while the existence of equatorial cur- 
rents tended. to give the marine animals a wider unbroken 
range than they now have. There were then striking contrasts 
of temperature between eastern and western continental shores, 
and variations of a few degrees were quite sufficient to produce 
very different climates. There are many of the same types in 
the deep waters of the Atlantic, the Pacific, and the Southern 
