80 REPORT — 1876. 



coast-like margin seen from Edinburgh or from Symington station on the Caledo- 

 nian line. But the surface of these Silurians was denuded before the Old Red 

 times, as Mr. Geikie has showed. Nay, valleys existed as now, and in the same 

 positions as now. At the present time the rivers flow in identically the same 

 valleys, in at least the cases of theNith, the Annan, the Lauder, and the Liddell; 

 and the boundaries of the areas are so well known that we can safely assert no 

 buried channel to exist such as we find on the tributaries of the Clyde. That the 

 channels were occluded iu glacial times we may take for certain ; that the obstruc- 

 tion has been washed away and the courses cleared is equally certain. The surface- 

 contours were not materially altered : so that the retreating ice left hollows in the 

 position of the old valleys. " But the case is quite ditterent when we deal with the 

 older rocks. Their succession is marked by unconformities and overlaps, which it 

 is impossible to picture as associated with full preservation of the surface-features 

 on which they were laid down ; and when the thickness comes to be as much as 

 1000 feet or more, and of that thickness a part at least made up of marine strata, 

 the relapse of all the streams to their old courses is an event of the highest impro- 

 bability. Mr. Topley has pointed out how the dip of strMa may under certain cir- 

 cumstances coincide with their thinning out to the margins of their area of deposit, 

 changes of angle in highly inclined strata pointing in the same direction. The 

 ordinary rule, of protracting strata and thus restoring their thickness over the adja- 

 cent high ground, is (iu the case at least of South Scotland) a method which imposes 

 on atmospheric denudation, even if aided by the sea, a most complicated task. 



Had time permitted, it might have been interesting to note the changing phrase- 

 ology regarding faults, and the pertinacity with which phrases involving the most 

 unsatisfactory and improbable causation continues to be used. L peast and down- 

 cast, upthrow and downthrow, displacement upwards or downwards — these it may 

 be said are of small importance ; they are only symbols. But, in the first place, 

 they are mischievous so far as they give students confused ideas with which to 

 contend ; and, in the second place, the continued acceptance of loose phraseology is 

 peculiar to geology. Even in metaphysics, where the subject-matter is much more 

 conveniently discussed in ordinary language, new terms are employed to a great 

 extent. But, important as I therefore regard these terms from the teacher's point 

 of view, the greater importance attaches to the accuracy of the notions which 

 imderlie our language regarding the processes and rates of deposit and denudation. 



So far as our present knowledge goes, we must accept it as certain that there is 

 some limit to the duration of the earth in the past. Neither philosophers nor as- 

 tronomers are agreed on the essential points of the problem ; nor have they consi- 

 dered all the possible changes in the position of the earth's axis, and in the rate at 

 which the earth loses heat. The limits hitherto prescribed are so discrepant that 

 we cannot as yet accept any as fixed. Neither have geologists so accurate a know- 

 ledge of geological processes that they can speak with confidence either of the ab- 

 solute or relative rates at which rock-formation has advanced. The geologist has 

 hitherto asked for more time, not because he himself was awai-e of his need, but 

 from a generous regard for the difficulties in which his zoological brother found 

 himself when lie attempted to explain the diversity of the animal series as the 

 result of slowly operating causes. The geologist aslied for more time simply 

 because he could form no just estimate of what was needed for the physical processes 

 with whose results he was familiar. But palseontological domination is now at an 

 end ; and the increasing number of geologists who are also competent physicists 

 and matJiematicians seems to mark a new school, which will strive to interpret 

 more precisely the accumulated facts. Such at least seems the history of the past 

 fifteen or twenty years. Such seems the direction in which speculation now tends ; 

 and in the foregoing remarks I have endeavoured faithfully to represent the drift 

 of our science. To many here present much of what I have said is already familiar ; 

 I therefore give place to the more legitimate business of the Section, looking to 

 receive elsewhere " such censures as may be my lot." 



