OVERTHRDST PHENOMENA. 25 



basin on the north and south and the approximate parallelism of the axes 

 of their foldings appear to indicate that the strains took effect on them at 

 approximately the same time. 



Complicated as is the structure of the Narragansett Basin, that of 

 Boston Bay is yet more involved. Although some minor folds may be 

 traced in it, and larger arches are fairly to be assumed, the area as a 

 whole appears to be much less massively and continuously flexed, and 

 more faulted, than that on the south. It is, moreover, far more generally 

 penetrated by dikes than is the Narragansett field. Its type of structure 

 seems to be between that of the last-named basin and that of the Connecticut. 

 It is probably owing to the relatively deep erosion of the Boston Basin 

 and the large amount of faulting in the orogenic work that it is so 

 difficult to recognize and determine the elements of folding which have 

 existed. 



OVERTHRTJ8T PHENOMENA. 



The phenomena of overthrusting which occur in the development of 

 mountain dislocation have of late been the subject of much profitable 

 incpxiry. It is therefore worth while to examine into the question of their 

 occurrence in this basin. As will be noted in the sections, the only 

 portions of the field where accidents of this nature seem to be indicated are 

 in the district extending from near Providence to the northern boundary of 

 the basin. The reason for this may be that only in this part are the 

 attitudes of the rocks near the margin sufficiently disclosed to make a close 

 interpretation possible. Particularly in the region about the Attleboros 

 tin- positions of the dislocated strata favor the view that the beds have been 

 at first folded and then thrust over, usually toward the less-disturbed centers 

 of the stratified rocks, so that a certain amount of migration of the beds has 

 been brought about. How far this has led to the disruption of the folds, so 

 that the masses which have changed place have been rent away from the 

 beds of which they originally formed a part, can not be determined from 

 the data now in hand. 



It is probable that the original margin of the Carboniferous rocks in 

 this basin was farther away from the center than it is at present. The 

 process of erosion, which has attacked the massive crystalline rocks of 

 the boundaries as well as the stratified interior parts of the trough, has 

 most likely lowered the whole of southeastern Massachusetts to the extent 



