PRESIDENT'S ADDRESS. Ixvii. 



proportion of silica is not much less than one part to from 

 50,000 to 100,000 parts of water. No chemical process is known by 

 which such minute proportions of silica could be directly precipitated 

 from a state of solution. Those animals and plants which have 

 the power of secreting a siliceous skeleton are the only agents 

 capable of separating even the minutest proportions of silica from 

 solution. The organisms which now form siliceous skeletons, and 

 on their death accumulate to form siliceous deposits, are the Diato- 

 macecv, the Radiolaria. and the Silicispongia, the first being plants 

 the second and third animals. The silica is separated by these 

 organisms in the colloid (opal) form, and afterwards is either 

 re-dissolved or passes into the more stable crystalline form. It 

 may be fairly concluded that the organisms which did the work of 

 separating the silica from the state of solution in the Cretaceous 

 seas were the same as are doing the same work at the present day. 

 Diatomacece are absent in the chalk, and until recent years J?adio- 

 larians were supposed to be so also, and in any of the pre-Tertiary 

 deposits. It is now known that they abound not only in the 

 Cretaceous rocks but in all deposits from the Silurian to the 

 present day. 



The late Sir Joseph Prestwich, in his presidential address to 

 the Geological Society in 1871, says "I think it probable that 

 some considerable portion of the deep sea-bed of the Atlantic has 

 continued submerged since the period of the chalk, and although 

 adaptable forms of life have been transmitted in unbroken suc- 

 cession through this channel, the immigration of other and more 

 recent faunas may have so modified the old population that the 

 original chalk element is of no more importance than the original 

 British element in our English people." Professor E. Forbes's 

 name is inseparably associated with the bathymetrical distribution 

 of marine life in Great Britain, and his clearly defined zones, the 

 Littoral, Laminarian, Coralline, and the region of deep-sea Corals, 

 from 50 fathoms to an undefined depth. Professor Loveh confirmed 

 the constancy of the Laminarian zone but denied that the deep-sea 

 zones could be compared with those in other areas, as they varied 



