STRATIGRAPHIOAL AND OROGENIC RELATIONS. 17 



Where the Narragansett Basin narrows, as it does in the southern third 

 of its length, the type of the folding differs somewhat from that above 

 indicated. In place of the folds on either side, with a less disturbed 

 middle field, the whole of the section is folded into a few great trough- 

 shaped undulations, with some minor irregularities. Yet the fact that the 

 strains entered the bedded rocks from the sides is shown by the character 

 of the bottom of the great North Aquidneck syncline. The form of this 

 part of the basin is tolerably well known by the mine workings in the 

 northern part of Aquidneck Island. These explorations show that the cen- 

 tral portion of this area has in a measure escaped^ the disturbing influences 

 which have perturbed the beds next the margins. 



Besides the evidences of stress which are shown by the extensive 

 dislocation of the stratified rocks of the Narragansett Basin, we must note 

 the equally characteristic marks of compression afforded by the interstitial 

 movements which the rocks have undergone. These changes of position 

 of the rock materials are exceedingly common in the metamorphic part of 

 the field (see fig. 6, p. 120) and for a short distance to the eastward, and are 

 readily observed wherever there are any data by which they may be judged. 

 Wherever the rocks lying near the eastern and western margins of the 

 basin contain organic fossils or pebbles, save of quartz, a slight examination 

 will in practically all cases show that those bodies have been more or less 

 elongated, the direction of their extension usually being on horizontal lines 

 which are approximately parallel to the neighboring margin of the basin. 

 At many points it is evident that the elongation of the pebbles or fossils 

 has been as much as 50 per cent of their original diameter on the given axis, 

 and sometimes it exceeds this amount. It commonly happens that the 

 distortion was sufficient to convert a circular disk lying in the axis of the 

 movement, and having a diameter of a foot, into an ellipse having a major 

 axis of 2 feet and a minor axis of 6 inches. In rare instances the alteration 

 of form appears to go much beyond this proportion. I suspect, indeed, 

 that at a few points it is obscurely traceable to three or four or even five 

 times the original length, but in these higher terms of the series fossils 

 become mere blurs and pebbles lose all semblance of their original 

 character. 



Although solid bodies like quartzite pebbles may often be found 

 stretched to the amount of 50 to 100 per cent, it is generally easy to see 



MON XXXIII 2 



