BOUNDARY OF THE BASIN. 129 



In the course of grading the bed of the Old Colony Railroad from 

 Brockton northward, since the field work for this report was finished, Car- 

 boniferous gray sandstones with fossil plant stems were exposed, according 

 to the studies of Mr. M. L. Fuller, 1 for some distance north of the main 

 boundary line on either side of that city. I am indebted to Mr. Fuller for 

 information concerning the boundary as drawn on the accompanying map. 



From Brockton to North River. — Outcrops of the stratified rocks here become 

 very infrequent, but enough are exposed to define the approximate position 

 of the boundary. 



shumatuscacant fault. — Between Brockton and Abington there are angles in 

 the border which indicate the existence of cross faults, one in the path of 

 Beaver Brook, the other, and more unmistakable case, that along the line 

 of the Shumatuscacant River in Abington. By this latter dislocation the 

 boundary line on the east is set northward for the distance of a mile. 



Through Rockland and thence eastward across Hanover to the north- 

 east corner of the basin, the boundary can be delineated only by drawing 

 a line south of the known exposures of granite and north of the first appear- 

 ance of Carboniferous rocks in the drift. The valley of Third Herring 

 Brook follows the boundary for a short distance, and the North River takes 

 an eastward turn across the line. 



From North River to Lakeviiie. — Along this line the boundary is, because of the 

 drift mantle, not accurately known. Granitite appears in Namasket about 

 4 miles west of the line, as heretofore represented on maps of the basin, 

 and it is not certain that this outcrop is a mere inlier. In the absence of 

 positive information on this point it is tentatively represented as a part of 

 the eastern granitic area. Between North Plympton and Halifax the basin 

 is also made to include an area of felsites, the exact limits of which are 

 unknown. The protrusions of the eastern margin into the basin in the form 

 of felsite in Plympton and of granitite in Middleboro are near enough to the 

 lines of anticlinal axes in this portion of the basin to lend support to the 

 hypothesis that the basement of the Carboniferous is exposed at these locali- 

 ties by the folding of that floor in conformation to the structure of the once 

 overlying strata. If this view be correct, we should expect to find the 

 lobate areas between the tongues of igneous rock forming synclines in the 



■Anew occurrence of C^vboniferous fossils iu the Narragansett Basin: Proc. Boston Soc. Nat. 

 Hist., Vol. XXVII, 1896, pp. 195-199. 

 MON XXXIII 9 



