TRANSACTIONS OF THE SECTIONS. 79 



tliiclmess in Wales as given in Jukes's Manual from the Survey data : for the Cam- 

 hrians we have from 2^000-28000 feet ; Silurians, Upper and Lower, not counting 

 breaks by unconformities, 20000. If denudation takes place at the rate of 1 foot 

 in 6000 years, and deposit at the same rate, we should have for the Silurians alone 

 120 millions of years needed. If, however, deposit takes place at the rate of 1 foot 

 in 14,400 years, 288 millions of years would be needed for the accumulation of the 

 surviving strata. It is obvious that the rate of deposit or denudation or both are 

 misunderstood. The stratified rocks equal in amount the material denuded ; if we 

 knew the total amount of denudation we should know, not merel)' the residuum 

 of rock open to our inspection, but the total amount of stratified deposits which had 

 been formed — or at least approximately ; for the deposit of materials removed is not 

 synchronous with their removal. Obviously these elements are not known, and 

 cannot be known, to us. Mr. Croll, who has investigated the question theoretically, 

 assumes that deposit and denudation take place in equal times, and assumes further 

 a uniform distribution over the whole or over a part of the sea-bottom. But Prof. 

 Geikie's table shows that, if we are to take averages as a safe guide, the land 

 is lowered at the rate of 2 feet in 6000 yeai*s. Moreover, if, as Mr. Croll points 

 out, deposit was less during the Glacial epoch, the process must have been more 

 rapid since ; and thus an iiTegularity is introduced which impairs the value of the 

 calculations. Prof. Hughes, in the brief abstract of his Royal-Institution address 

 which alone I have had the opportunity of seeing, contests the validity of any esti- 

 mates of time on the basis of our existing knowledge. 1 do not mean to enter into 

 this question ; but I may be allowed to remark that any conclusions founded on mean 

 thickness of sedimentary formations are of no value. It is not the time necessary 

 for the building-up of a mean thickness, but that necessary for the formation of the 

 maximum thickness in particular regions, which we have to consider. 



If the Laureutian rocks and their equivalents are to be regarded as the earliest 

 stratified deposits, or, rather, if there is no reason for believing that they were pre- 

 ceded by other stratified rocks, the relation of Huxley's homotaxis to any classifi- 

 cation of strata having the Laurentians as a fixed point is worth investigating. 

 The universal dift'usion of species in the earlier strata was first the accepted creed 

 of geologists. Then it was denied, though the language of the earlier faith con- 

 tinued cm-rent. Again we return towards the doctrine of extensive simultaneous 

 diffusion, but under a very much modified form. The 'Challenger' reports bear 

 testimony to the wide distiibution of forms in the deepest oceans ; and when we 

 turn from these and compare the lists of fossil species so found widely distributed, 

 it appears that here again we have oceanic foims, or at any rate those found in such 

 limestones as are safely assigned to a deep-water origin. Ramsay has shown that 

 the continental epochs in Western Europe overlasted considerable periods of time. 

 The antiquity of the Atlantic and Pacific is certain ; even their primitive character 

 is possible. Thus there are two conditions, land and deep-sea, reasoning regarding 

 which must be quite difi'erent from that applicable to the intermediate conditions. 

 It is exactly these intermediate states which present practical and speculative 

 difficulty. Theories which account for mountains and oceans fail to explain the 

 " oscillations " which were wont to be appealed to when terrestrial and marine sur- 

 faces succeeded each other. But the assumed movement of the land is by no means 

 a certainty ; and, as in the kindred case of faults, we need terms which shall be neu- 

 tral, whether the land has moved upwards or the sea shiunk downwards. The 

 terms Palajozoic, Mesozoic, and Cainozoic have long held their places from the re- 

 luctance to disturb established nomenclature, as well as from the difficulty of in- 

 venting appropriate substitutes ; but if retained at all, we know now that the rela- 

 tions they represent are not the same for the terrestrial, the deep oceanic, and the 

 intermediate areas, any more than the life is the same under those three conditions. 



I have once before called attention to a grave difficulty in the physical geography 

 of Scotland ; and as Mr. Seeley has since then raised the same question without ob- 

 taining an answer, I would again state the case, as one which seems to involve the 

 revisal of some definitions. 



The Silurian hills of South Scotland are commonly said to have been covered by 

 Old Red Sandstone and even by Carboniferous strata — patches of these rocks being 

 met with on the south side of the fault which defines these hills, with their abrupt, 



