82 Horner's Geological Address. 



rocks which contain the oldest forms and first traces of organic 

 life, that from the highest beds of the Lower Silurian rocks 

 to the lowest deposit in which organic remains have been 

 found, there had been no great variation in the circumstances 

 under which these beds were deposited, although there is evi- 

 dence of a long duration of time, in which gradual changes in 

 animal life took place, some species diminishing in numbers, 

 others becoming extinct, others continuing to exist through- 

 out the whole range, and a few appearing in the lower por- 

 tion of these beds, which, from a marked general change of 

 forms, are classified as the Upper Silurian rocks. This view 

 you will see developed in the address delivered by Sir R. 

 Murchison from this chair four years ago,* where he states, 

 that the conventional line that had been drawn between the 

 Lower Silurian and the Cambrian rocks beneath them had 

 no longer any reference to strata identified by distinguishing 

 organic remains ; for the same fossils are found in strata on 

 each side of that demarcation. He also stated, on the same 

 occasion, that '' the zone of fossiliferous strata characterized 

 by the Lower Silurian Orthidse are the oldest beds in which 

 organic life has been detected," and his belief that " many 

 of the subjacent rocks, sometimes even when in the form of 

 gneiss, mica-schist, talc-schist, chlorite -slate, &c., are nothing 

 but metamorphic rocks, in less altered parts of which the 

 same typical fossils are observable." In his recent work on 

 Russia he asks the questions, " Can we lay open the earliest 

 vestiges of animal life, and amid palaeozoic forms trace back- 

 wards primeval history to a protozoic type ? Can we sepa- 

 rate such protozoic strata from those which went before 

 them, and were deposited ere life had been breathed into the 

 waters .^"t To the latter question I am disposed to answer, 

 that the mere negative fact that we have not yet discovered 

 traces of organized bodies in the lowest strata, certainly 

 does not warrant the inference that no living thing had yet 

 existed, or our saying, that any strata were deposited " ere 

 life had been breathed into the waters.'* If these strata con- 

 tain a particle of undoubted detrital matter, a grain of rolled 



* Proceedings of the Geol. Soc, vol. iii., p. 642. 

 t Eussia and the Ural Mountains, &c., vol. i., p. 1. 



