•440 Ilobbs — Geological Structure of the 



Rogers brothers, who by their explanation of Appalachian 

 structure on the basis of folding alone, not only established a 

 new type which bears that name, but developed methods which 

 have been left as a legacy to their successors. 



In Europe, on the other hand, the principles so long ago 

 worked out by Kjerulf, Sedgwick, De La Beche, John Phil- 

 lips, and especially by Daubree* have been utilized to such an 

 extent that the recognition of systems of faults as important 

 elements of crustal deformation may be truly said to be the 

 most marked line of cleavage which separates European struc- 

 tural studies from American. It has been especially the work 

 of Suess to correlate these scattered studies and to show that 

 the lineaments of the continents are lines of normal faulting, 

 between which great orographic blocks have been depressed by 

 different amounts. His monumental work upon The Face of 

 the Earthf has been the greatest interpretative work upon 

 structural geology of the past quarter century, and marks an 

 epoch in the history of the science. In the Principles of 

 North American pre-Cambrian Geologv, the most comprehen- 

 sive American treatment of rock deformation, Van Hise 

 devotes 75 pages to a consideration of deformation by folding, 

 whereas 10 pages suffice for a treatment of jointing and fault- 

 ing inclusive of thrust faults which are connected with folds. 

 There is no doubt that the proportion of space thus devoted 

 to joint and fault structure is far beyond what its consideration 

 by American geologists would warrant. 



Inadequacy of Hypothesis of Folding. 



As resjDects the study of the southwestern ISTew England 

 area, it is, I believe, time to admit that the basal assumption 

 of a deformation by folding alone, which has now been given 

 a trial by the Survey during a period of nearly twenty years, 

 has proven entirely inadequate. Where belts of limestone are 

 present to serve as guides in the areal and structural work, the 

 order of superposition of formations, and in a very general 

 way the geological structure, have been made out with a 

 reasonable degree of certainty. In the broad belt of schists 

 and gneisses which lie to the eastward of the limestone, on the 

 other hand, it has thus far been found impracticable to dis- 

 cover any structure regarding the presence or absence of which 

 any two of the w r orkers can agree. This is true in spite of the 

 fact that the geologists in the field have acquired by long 

 experience a familiarity with the varied petrographical types 

 of the province and with the apparent structures developed in 



* Daubree, G^ologie experimentale, pp. 304-306, Paris, 1879. 

 f Antlitz der Erde, two vols., Prag and Leipsic, 1885. 



