44 INTRODUCTION. 



with siliceous Sponges, Sea-urchins, and Crinoids. We learn, 

 therefore, from this, that contemporaneous deposits not only 

 do not necessarily contain the same fossils, but that, if 

 widely separated geographically, they may be characterised 

 by wholly dissimilar assemblages of organisms. 



It may happen, again, as pointed out by Sir Charles 

 Lyell, that deposits belonging to different geographical and 

 zoological provinces may, as regards space, be nearly ap- 

 proximated, and, as regards time, may be actually con- 

 temporaneous, and yet may not contain any fossils in com- 

 mon, or only a very few. If, for example, any sudden 

 upheaval were to lay bare what is now the floor of the Red 

 Sea, together with that of the Mediterranean, we should find 

 the two areas to contain deposits actually synchronous as 

 regards the time of their deposition, and very near to one 

 another in point of distance, and yet containing, upon the 

 whole, entirely distinct groups of organic remains. We 

 learn, therefore, from this, that owing to the existence of 

 geographical barriers, it is possible for contemporaneous 

 deposits to be found in close contiguity, in a single region, 

 and yet to contain very different fossils. 



Again, we know from the researches of Professors Car- 

 penter and Wyville Thomson and Mr Gwyn Jeffreys, that 

 deposits may be formed, side by side, in a single ocean, and 

 may yet differ from one another altogether, both in mineral 

 characters and in their included fossils, though strictly con- 

 temporaneous in point of time. Thus, in parts of the deep 

 Atlantic where the temperature of the bottom water is com- 

 paratively high, we have the calcareous deposit of the ooze, 

 abounding in Foraminifera, Sponges, and Echinoderms. In 

 certain other areas in the same ocean, and in comparatively 

 close contiguity with the preceding, we have the tempera- 

 ture lowered by cold currents, and we find a sandy deposit 

 in process of formation, with a fauna much more scanty 

 than that of the ooze, and wholly distinct from it. We thus 

 learn that sedimentary deposits may be strictly contempo- 

 raneous, and may be placed very near to one another in 

 point of distance, and yet may contain very different fossils. 



Lastly, synchronous deposits necessarily contain wholly 





