484 Eev. J. F. Blake- — Aggregate Deposits and Zones. 



different periods may have been brought together, it is not necessary 

 that an aggregate should contain fossils of more than one epoch, the 

 etymology of the word connoting only the assemblage of materials 

 that have been moved horizontally, like a flock of sheep, over the 

 surface of the ground. I would also propose to distinguish by name 

 the two classes of fossils preserved, as noted above, in different ways. 

 The ordinary fossils which are buried where the animals died or fell 

 to the bottom, we may call autocJithonoiis ; and those which have been 

 drifted to their final resting-place by currents, lieterochtlionous. The 

 shells which lie scattered on the sands by any seashore, when buried 

 will produce autochthonous fossils, while the heaps of dead shells 

 which are crowded together in certain localities, as the island of 

 Harm, Morte Bay, and the estuary of the Thames, will produce 

 heterochthonous fossils.^ 



From these definitions it follows that the fossils of an aggregate 

 are essentially heterochthonous, though they may be accompanied by 

 others which are practically autochthonous. It follows also that we 

 may expect aggregates to be exceptionally fossiliferous, and con- 

 versely, when we hear of a bed being exceptionally fossiliferous 

 we may suspect that it is likely to be of an aggregate character. 

 But the most essential point to notice is, that when a bed is an 

 aggregate the fossils in it have not been buried pari passu with 

 the formation of the deposit, but existed before it commenced to be 

 formed, so that we have no proof that the fossils it contains belong 

 to the same age as the deposit itself, . or that they belong to any 

 single age. This would, of course, be self-evident if the fossils 

 were recognized as remanies. But it is not necessary that a 

 heterochthonous fossil should have been previously buried in 

 a deposit which became sufSciently hard to make a pebble ; it may 

 have lain on the sea-floor, even for geological ages, uncovered by 

 deposit, or so slightly covered that it is washed out again by the 

 current when this begins to flow, and is finally buried in the finer 

 mud brought by this current. The extinct bones and teeth which 

 have been dredged up from the bottom of the Atlantic prove the 

 possibility of this. 



These extinct sub - Atlantic remains have also a remarkable 

 peculiarity about them which is very instructive in relation to 

 aggregates — they are phosphatized. If phosphatization be connected 

 with the decay of marine organisms it is plain that lying on the sea- 

 bottom must be more favourable to the process than being imbedded 

 even in a porous deposit. No doubt phosphatization can be produced 

 in the heart of a deposit, many autochthonous fossils having phos- 

 phatic aureoles — which may have been produced either during the 

 progress of the deposition or subsequently — but, nevertheless, in 

 a large number of cases, when the nature of the deposit, 

 independently of its chemical composition, would lead us to call 

 it an aggregate, we find that it contains phosphatic nodules, and 



1 At the reading of this paper Mr. Lomas instanced also some drifts of dead shells 

 now forming in the Irish Sea, and gave the interesting information that the deposit 

 had been partly phosphatized. 



