20 GEOLOGY OF THE NARBAGANSETT BASIN. 



filled with their bedded rocks, lay fields of crystalline materials which had 

 been compacted by previously administered pressures and the accompany- 

 ing metamorphism until they had been brought to the most rigid or the 

 best thrust-transmitting state to which rocks may attain. The result was 

 that the compressive strains were transmitted through these ancient close- 

 knit blocks of strata to take effect on the frailer materials which were 

 inclosed in the troughs between their edges. The assumed conditions are 

 in a diagrammatic way represented in the accompanying figure (fig. 1). 



RESULTS OF THE ACTION OF OBOGEISTIC FORCES. 



So far as can be determined by the evidence which has been found, 

 the orogenic movements which flexed and fractured the Carboniferous rocks 

 of the Narragansett Basin did not affect in a similar manner the more 

 ancient formations which border the area. There are, it is true, certain 

 faults intersecting the margin, particularly on the northern side of the area, 

 which cut through the Carboniferous beds and the fundamental complex 

 alike, but there is no evidence that these faults extend very far beyond the 

 margin or that folds attended the formation of the rifts. Any system of 

 anticlines and synclines affecting the more ancient beds would, we may 

 fairly presume, have left their marks in the distribution of the deposits as 

 they appear on the present surface. The several groups of metamorphosed 

 beds would appear in parallel bands, an arrangement which they do not 

 exhibit. 



The only distinct feature in the ancient compact rocks which can be 

 attributed to compressive action is the system of shearing planes which 

 have been extensively developed in the massive rocks on the margin of the 

 basin. Where these have been developed in the granitic rocks they have 

 given the latter a gneissoid aspect. These secondary structures are most 

 distinctly marked near the contact borders on the east and west, and appear 

 to diminish as we pass a few miles from the present margin of the Carbon- 

 iferous field. The existence of this class of distortions in the rocks, along 

 with the general lack of evidence of folding, points to the conclusion that 

 the pressure affected these compact formations in a way different from 

 that in which it affected the stratified beds of the basin. It may be that 

 the yielding in the interstitial movements accomplished a certain reduc- 

 tion in the length of the sections even when the materials were too rigid 



