32 GEOLOGY OF THE NARRAGANSETT BASIN. 



It should be said that the mainspring of the development which has 

 taken place in these basins is the crustal strain which manifests itself in 

 mountain building. Where this strain, as in the ordinary conditions of 

 mountain growth, such as existed in the West Appalachians, takes effect 

 on uniform, horizontal, little eroded strata, the action appears to result in 

 the formation of elongated, more or less accurately parallel ridges, such as 

 are exhibited in the Jura or the Alle^ghenies. Where, on the other hand, 

 as along the Atlantic coast, the crust is composed of ancient massive rocks 

 in which deep valleys have been excavated, the orogenic strains result in 

 the deformation of the patches of stratified rocks which may have been 

 accumulated in the great valleys during the periods of subsidence. 



The dislocations of the Atlantic coast basins clearly indicate that the 

 stress which has caused them was what we may term quaquaversal — that is 

 to say, it has acted in several directions around the greater part of the hori- 

 zontal circle. This behavior of the crustal stress is quite different from that 

 which we find exhibited in normal mountains; there, as before remarked, the 

 relief has been obtained by the formation of ridges and furrows, the axes 

 of which are nearly parallel to the same great circle. Although the amount 

 of this parallelism has usually been much exaggerated, there can be no doubt 

 as to its substantial existence. However, it seems unwarranted to suppose 

 that the axial relation of the ridges is due to the existence of a strain acting 

 in but one direction. All that is required to produce the result is either a 

 certain predominance in the value of the strain in a particular versant, or, 

 what comes to the same thing, a greater tendency to yield along one set of 

 lines. Instances of this may frequently be seen in the wrinkling of veneers 

 or in sidewalks which have been covered with some plastic materials. With 

 a simple device the cream on a pan of milk may be made to show the 

 effects of the same general principle, where the giving way takes place 

 rectilinearly, though the difference in pressure in the several axes of the 

 circle is but small. Moreover, even in the most nearly parallel mountains, 

 there are generally to be found cross folds which show very clearly that 

 the strain has not been uniaxial. 



The foregoing considerations lead us to infer that the diversity of axes 

 in the elevations produced in the singular group of antecedent basin folds 

 which we are considering has not been brought about by a class of strains 

 differing as regards their distribution from those involved in the formation 



