160 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



extremity of that continent. It is also more than probable, from the 

 information we already possess, that they exist in Australia. The 

 rocks were known, and had been partially described, but they were 

 not understood ; they were known mineralogically, and deposits sepa- 

 rated by great intervals of time were classified together under the 

 vague, uncertain, general term of grauwacke, or grauwacke slate, or 

 clay-slate. The clear development of the system, and lucid descrip- 

 tions of the normal types in the Silurian region of Britain, dispelled 

 the obscurity that hung over the history of these ancient beds ; and 

 now geologists are at work in all countries, making out the great fea- 

 tures of resemblance, and registering those variations in mineral and 

 fossil contents, dependent on geographical position and other local 

 causes, which are found to prevail more or less in all formations. 



It appears to be now the opinion of those geologists who have 

 most carefully and extensively studied the sedimentary 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 deposits 

 in which organic remains have been found, there had been no great 

 variation in the circumstances under which these beds were depo- 

 sited, although there is evidence 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 

 throughout 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 iden- 

 tified 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 charac- 

 terized by the Lower Silurian Orthidfe 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 ob- 

 servable." In his recent work on Russia he asks the questions, 

 " Can we lay open the earliest vestiges of animal life, and amid pa- 

 laeozoic forms trace backwards primaeval history to a protozoic type ? 

 Can we separate such protozoic strata from those which went before 

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

 To the latter question I am disposed to answer, that the mere nega- 

 tive 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 ani/ strata were de- 

 posited " ere life had been breathed into the waters." If these 

 strata contain a particle of undoubted detrital matter, a grain of 



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

 t Russia and the Ural Mountains, &c. vol. i. p. L 



