CONTEMPORANEITY OF STRATA. 1 9 



ooze of the deep Atlantic would contain innumerable Foram- 

 inifera, along 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 approximated, and, 

 as regards time, may be actually contemporaneous, and yet 

 may not contain any fossils in common, 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 Mediter- 

 ranean, we should find the two areas to contain deposits actu- 

 ally synchronous as regards the time of their deposition, and 

 very near to one another in point of distance, and yet contain- 

 ing, 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 de- 

 posits 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 Carpenter 

 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 contemporaneous in point 

 of time. Thus, in parts of the deep Atlantic where the tem- 

 perature of the bottom water is comparatively 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 temperature 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 contemporaneous, 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 dif- 

 ferent fossils, if one has been deposited by fresh water, and the 

 other has been laid down in the sea. The fresh-water deposits 

 of one period are obviously contemporaneous with the marine 

 formations of the same period, and they may not be far 

 removed from one another in point of distance, but they must 

 contain altogether different organic remains. The former will 



