FOLDS AND FAULTING. 461 



applies ; arch and trough following each other in regular 

 sequence, such as the arches of the Rockies and the Ap- 

 palachians, with the simple trough of the Mississippi between 

 them, or the two depressions of the Atlantic, with its central 

 ridge, on whose steep gradients are situated the volcanic 

 islands of St. Helena and Ascension, the Madeiras, and 

 the Azores. And so on through the two-ridged continent 

 of Africa, with its central table-land between ; the Indian 

 Ocean, with its double trough ; and, finally, the Australian 

 continent, flanked by its two ranges, the eastern and the 

 western. 



But the complexity proves even greater, the earth 

 being regarded as triple folded in two directions ; thereby 

 explaining the more prominent of its geographical 

 features. Geography has given convincing answers 

 to questions first raised on geological bases, and we are 

 led step by step from the world-wide conception of the 

 fold through the minuter, but no less striking, foldings, as 

 represented in highlands and mountains, down to possible 

 changes of ultra-microscopical dimension. 



But the extension of this theory, explaining, as it does, 

 to the satisfaction of its author every great depression and 

 elevation on the earth's surface, demands a far wider appli- 

 cation in dealing with the past history of our globe, There 

 again Professor Lapworth has not hesitated to apply his 

 method, and he postulates that, in opposition to the views 

 held by many distinguished observers, affirming the absolute 

 permanence of ocean basins and continental structures, there 

 is strong evidence that our islands, for instance, have under- 

 gone successive stages of elevation and depression, the 

 alternation between the two extremes being marked by 

 periods of great volcanic activity, as, for example, in Ordo- 

 vician and Miocene times. 



A question of great importance now presents itself, viz., 

 whether these foldings are the result of mere accidental 

 disturbance, possessing no fundamental sequence ? and Pro- 

 fessor Bertrand (13), in seeking an answer, has inaugurated 

 an interesting discussion. 



The three main problems which he has endeavoured to 

 solve in his work on the Paris Basin are : — 



