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 owino- to the variable nature of the bottom, it does not 

 follow by any means that the same process is going on sjn- 

 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 

 palaeozoic times is very uncertain. All the evidence thus far 

 tends to show that the deep-sea fauna originated at the close of 

 the palaeozoic times, for many of the genera which we have been 

 led to consider as deep-sea types in the mesozoic 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 faunae, like all marine 

 faunae, are essentially dependent upon the nature of the bottom ; 

 in former times, globigerinae and siHceous sponges must have 

 flourished on calcareous ooze ; mollusca and annelids character- 

 ized muddy bottoms, while polyps, gorgoniae, 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 sfive 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 suf&cient to produce 

 very different climates. There are many of the same types in 

 the deep waters of the Atlantic, the Pacific, and the Southern 



