FOLDS AND FAULTING. 403 



giving an example of inversion and intrusion, he adds : 

 " Until some rational theory is produced of the mode in 

 which an overlying formation, hundreds of square miles in 

 extent, and thousands of feet in thickness, can have been 

 metamorphosed, whilst the underlying formation, of equal 

 thickness, and scarcely less in extent, has escaped, we shall 

 be justified in admitting inversions and extrusion equal to 

 those in the Alps ". 



If somewhat vaguely expressed, it is evident that Nicol's 

 thoughts were already tending in the direction of a truer 

 solution, but the answer given by Murchison and Geikie (7) 

 appeared so conclusive and overwhelming that further dis- 

 cussion was silenced. Pure stratigraphy had triumphed, 

 and tectonic geology slumbered, in so far as the British 

 Islands were concerned. Meanwhile, on the Continent and 

 in America, materials were being accumulated, destined 

 eventually to lead to conclusions of the highest importance ; 

 but before the new views could be brought into prominence, 

 one of the conflicting hypotheses had first to gain a com- 

 manding position. Only with the abandonment of Von 

 Buch's hypothesis of elevation by Plutonic upheaval, and 

 the recognition that Elie de Beaumont's brilliant theories 

 of mountain structure were untenable, could lateral pressure 

 due to contraction of the earth's surface become a serious 

 factor in the discussion of foldings. Hall, De la Beche, 

 Prevost, Pratt and Fisher had been largely instrumental in 

 bringing about this valuable result, whilst Daubree had 

 attacked the question from a practical standpoint, and had 

 carried out a large number of experiments having a most 

 important bearing on the point at issue. In 1875 Dana, 

 adopting a line of thought parallel to that already pursued 

 by Babbage and Herschel, pointed out the important re- 

 lation existing between mountain formation and the previous 

 deposition of thick layers of sedimentary deposits, the posi- 

 tion of the future mountain range being first marked out by 

 the slow formation of a geo-synclinal, the deposition of 

 sediment being concurrent with the progress of depression. 

 As the geo-synclinal descends, a geo-anticlinal must be 

 formed on one side, or possibly on both, but in the xA.ppala- 



