26 GEOLOGY OF THE NARRAGANSETT BASIN. 



of several hundred, perhaps to the depth of 1,000 or more, feet, the result 

 being that, as the old bordering walls had an erosion slope, the margin is 

 at present perhaps some miles nearer the center than it was when the folding 

 was done. It has also been noted that the folding and other evidences of 

 stressing which the rocks present diminish, except on the northern border, 

 as we pass from the margin toward the interior of the Carboniferous area. 

 These considerations lead us to see that the portions of the beds which were 

 most dislocated have been removed by the erosion process, which, as is 

 shown by the planed-down character of the surface, has undoubtedly been 

 very active in this part of the continent. It is therefore not unlikely that 

 great migrations of strata took place at the time of the disturbance, the 

 indications of which have been entirely lost to us by the process of decay 

 and removal of the beds which were involved in the movements. In a 

 word, the zone where we might most naturally suppose the overthrusting 

 actions to have taken place has in large part disappeared, at least so far as 

 the superficial beds are concerned. 



From the conditions presented by this basin there is reason to believe 

 that the rupture and horizontal displacement of folded strata would be more 

 likely to occur here than in the ordinary instances of mountain folding. 

 Under the usual circumstances, where the contracting impulse affects a 

 large extent of country, influencing all the rocks alike, the relief effected 

 by the corrugations of the strata is apt to be equal in all parts of the area 

 subjected to the movement. When, however, as in the Narragansett dis- 

 trict, the strains were most applied on the margins of the field, where the 

 tensions developed in a wide extent of country were localized in a narrow 

 zone of relatively weak strata, we must expect the highest type of distortion 

 and rupture that occurs in mountain- folding Avork. 



Such overthrusting as has occurred in the Narragansett Basin appears to 

 a great extent to have been begun by folding, the arches being raised to a 

 considerable height. These arches appear to have collapsed, as all com- 

 pressed arches tend to do. 



Overthrusting action appears to be most probable in the region between 

 the villages of North Attleboro and South Attleboro, where the relation of 

 the red Wamsutta series to the gray rock on the south requires the supposi- 

 tion of this movement. So far as has been observed in the few traceable 



