18 GEOLOGY OF THE NAEEAGA]S"SETr BASIK 



that the matrix in which they are embedded has been much more extended 

 than the more rigid inclusions. This is shown by the fact that near each end 

 of the axis of the pebble in which the elongation has taken place there is 

 often a slickensided surface, showing that the matrix pulled away from the 

 sides of the stone and shpped by them, while at the very end we note the 

 existence of a vein deposit which is also sHckensided after the manner of 

 most veins, the structure showing that the movement which pulled the matrix 

 away from the inclosed fragment operated slowly and by successive steps. 

 It often happens that pebbles, especially those of large size and of the more 

 rigid varieties of stone, show no distinct signs of elongation, and yet they 

 have these appended veins and the slickensides, indicating that the more 

 plastic matrix has yielded to the pressure which has been imposed upon it. 



There are certain features in the distribution of elongated rock masses 

 which appear to throw some light on the questions we are now considering. 

 In, the first place, we note that in the Narragansett Basin, and, so far as my 

 observations go, in the other basins of the East Appalachians as well, the 

 plastic migrations of the strata, as shown by the distortion of fossils and 

 pebbles, are decidedly more common in the marginal parts of the several 

 fields. These elongational phenomena are strikingly manifested at many 

 points in the southern portion of the Narragansett area. In the central 

 part of the area there are extensive tracts of conglomerate where a close 

 examination has failed to indicate either that the pebbles were stretched or 

 that the matrix was forced by these inclusions. This goes to show that the 

 force which affected the stratified rocks came upon them as a thrust hori- 

 zontally transmitted from the field of crystallized rock on either side. 



The other point concerns the regional distributions of plastic migration 

 of rock. This phenomenon seems to be of common occurrence in the East 

 Appalachians from New Brunswick southward It is excellently shown at 

 the Oobscook Basin, about Mount Desert, and in the Boston Basin; in the 

 basins to the southward it is more scantily exhibited. Curiously enough, 

 however, this feature appears to be prevaiHngly lacking in the rocks of the 

 West Appalachians, notwithstanding the deposits of that section appear to 

 have been on the average about as much dislocated by folds and faults as 

 have those of the eastern section It is not clear to what the greater plastic 

 movement of the eastern rocks has been due. It may, however, be sug- 

 gested that such movements depend upon the compression of rocks while 



