58 SECTIONAL ADDRESSES. 



modern mountains of Switzerland, combined with elevation. We meet 

 witli it in tlie much more ancient and less exalted mountains of our own 

 country, combined with unconformity. Such unconformity speaks to us 

 of elevation brought low by erosion, coupled in many cases with actual 

 subsidence. The evidence, carefully considered, justifies us in restoring 

 to the ruined heights an original grandeur comparable to that of their 

 proud successors. 



I have just referred to two of the fundamental conceptions involved 

 in our subject — lateral compression and unconformity. Their significance 

 was early appreciated in the study of the Southern Uplands of Scotland. 

 In 1812 James Hall suggested lateral compression as the cause of the 

 ' convolutions ' of the Silurian strata visible in the coastal cliffs of Berwick- 

 shire. He spoke of ' horizontal thrust,' and imitated the observed effect 

 by the sideways crumpling of a pile of cloths. As for unconformity, its 

 critical discussion represents one of the main achievements of Hall's 

 master, James Hutton, Father of Modern Geology. Unconformity is a 

 comprehensive word used by geologists to express an erosional gap in the 

 stratigraphical sequence. Some unconformities are obscure and debatable ; 

 but unconformities that succeed periods of mountain folding furnish most 

 impressive spectacles. Hutton long searched the Southern Uplands for a 

 contact of the fiat Old Red Sandstone and the steeply folded Silurian 

 greywackes. His scientific imagination pictured in advance the relation- 

 ship of the two formations, and he felt that its demonstration ' would 

 add great lustre ' to his Theory of the Earth. In 1787 he found his 

 expectations fully realised in the banks of the River Jed, where horizontal 

 Old Red Sandstone covers an eroded surface that truncates the steep 

 bedding of underlying greywackes. 



Hutton saw in the Jed exposures a buried mountain chain in process 

 of disinterment. The mountain rocks have just been reached by the river, 

 and are therefore restricted to the valley bottom ; but they possess an 

 inherent quality which will presently lead to a reassertion of something 

 of their old predominance in landscape. The compressional forces 

 responsible for mountain building tend to indurate the materials upon which 

 they operate. They therefore exercise a potent though indirect influence 

 upon the development of scenery, wherever and whenever folded mountains 

 appear at the surface. Let us always remember that the beauty which 

 characterises the mountain exposures of Britain has more to do with 

 resurrection than survival. Most, if not all, of the folded mountains of 

 our islands have been beneath the sea and covered by unconformable 

 deposits at some period since the day of their plication. They owe their 

 partial reappearance to subsequent upheaval and denudation. Erosion, 

 busy at first, has stripped away much of the comparatively unresistant 

 cover ; now it lingers and permits the re-exposed mountain rocks to stand 

 for a while as uplands overlooking adjacent plains. 



The same general story holds in countries other than our own. 

 Accordingly, certain old mountain areas, such as the Highlands of 

 Scotland and the Harz of Germany, were recognised by their inhabitants 

 without help from geologists ; but when Suess and his disciples came to 

 synthesise mountain chains from exposed fragments, they naturally had 

 to supply names for their discoveries. 



