254 REPORT OF MEETINGS FOR 1902 



surface features in a district like that under consideration. 

 Likewise we may leave entirely out of account cataclysmal 

 action, of any kind soever. Nothing else of importance besides 

 the quiet and gentle action of subaerial forces has been 

 concerned in removing the rock which formerly occupied the 

 place where now are the valleys. All the material has been 

 gradually removed from that area, and little by little, in the 

 course of long ages, been transferred from the land to the sea, 

 where it now lies spread out upon the sea bottom in the 

 form of layers of mud. sand, and silt. 



This statement about the valleys may be put into another 

 form. We may picture to ourselves a time in their history 

 when the present deep valleys had no existence, and when 

 the rivers that have shaped them flowed at levels much 

 nearer to those of the summit plain than they do now. And 

 if it had been possible to record tlie successive stages in the 

 shaping of the valleys, by inspecting the work at different 

 periods, we should have found that at the end of each long 

 interval of time the river had cut its way down a little 

 deeper, and had widened its valley a little more, as time had 

 gone on. And so, in the course of long ages, of which the 

 whole of the historical period forms but a very small part, 

 the depressions have been carved by rain and rivers into 

 their present form. 



The history of the summit plain calls for some fuller 

 explanation ; but this cannot well bo given in a form that 

 would be intelligible to those who are not versed in geological 

 matters until an outline of the geological history of the district 

 as a whole has been laid before the reader, which, accordingly, 

 will be given here. 



The oldest rocks in the district rise to the surface to the 

 north of Peebles. They belong to the geological group known 

 as the Ordovician rocks. The lowest of these consist of a 

 group of volcanic rocks, of high geological antiquity, which 

 appear to have been erupted beneath the sea. These are 

 followed by some beds of what are now chert and jasper; 

 but when these are carefully examined under the microscope 

 they are seen to consist chiefly of some lowly marine organisms 

 called Eadiolaria, whence the deposit in question is called 

 a radiolarian chert. The feature of interest in connection 



