554 SCIENCE PROGRESS 



isostasy with mountain building. H. F. Reid shows that 

 many great fold-mountain chains are bounded by huge faults, 

 and contain within their limits widespread dissected peneplains 

 which post-date the folding. Hence he believes that the 

 folding took place mainly at low levels, and was succeeded by 

 a period or periods of peneplanation. Subsequent vertical 

 movements uplifted the chains to their present altitude. Thus 

 the objection to the principle of isostasy based on the belief 

 that mountain chains were elevated by horizontal compression 

 is rendered invalid. The larger vertical movements are due to 

 changes in the density of the underlying crust, concerning 

 the cause of which we can only speculate. 



H. O. Wood suggests the growth of batholiths as a possible 

 cause of mountain building which can be appealed to quite 

 independently of tangential pressure. He also believes that 

 " the weakening and strengthening of adjacent elements of 

 the outer shell by readjustments brought about by erosion or 

 deposition under effective isostatic control maypermit tangential 

 stress to be generated, sustained, and transmitted to weak 

 regions, so as to build mountains, ... so long as any effective 

 compacting of the interior of the earth continues from any 

 cause." 



Sir S. Burrard is now doubtful that the hypothesis of 

 isostasy excludes the hypothesis of the origin of mountains by 

 horizontal folding. He argues that if the excess loads of 

 delta material are automatically compensated in depth (as 

 they are), why should not loads locally accumulated by folding 

 be also compensated ? 



From mechanical considerations A. C. Lawson shows that, 

 for a real overthrust, the limiting length of the overthrust 

 prism in the direction of the force applied is between 20 and 

 30 miles when the thrust plane is nearly horizontal, and about 

 half this amount if the thrust plane dips at 20 degrees. 

 These values agree well with those actually observed in over- 

 thrust regions such as north-west Scotland and Scandinavia. 



A. C. Lane makes the important suggestion that it is the 

 weight of rock per unit volume that matters in certain geolo- 

 gical problems rather than the specific gravity of the rock 

 powder as usually found. In questions of isostasy this is 

 especially the case in computing the weight of crustal columns 

 made up partly of recent water-logged (or oil-logged) deltaic 

 and other sediments. 



By a consideration of the average density of igneous rocks 

 within certain large and small regions H. S. Washington shows 

 that the average density of the igneous rocks of a region varies 

 in the opposite sense as the average altitude. This result 

 harmonises with, and corroborates, the theory of isostasy. 



