r East and West System. 535 



Plastic Clay Formation on Martha's Vineyard seems to have under- 

 gone, may have resulted from the protrusion of granite. If so, that 

 granite was probably a portion of the east and west system under de- 

 scription. According to these data, the epoch of its elevation must 

 have been subsequent to the deposition of the Plastic Clay, and con- 

 sequently later than the epoch of the northeast and southwest system. 

 But I regard this evidence as little better than conjecture ; and there- 

 fore, I am greatly in doubt as to the period when this system was ele- 

 vated. 



It is natural to enquire whether any traces of this system can be 

 discovered beyond the limits of Massachusetts. The only means 

 I have of forming any conjecture on the subject, consist of geo- 

 graphical maps, which give the direction of chains of mountains : 

 and even in this respect they are so imperfect, as to the more unex- 

 plored parts of our continent, that they leave one in much doubt. I 

 will only, therefore, say, that on our best maps, a range of mountains 

 extends along the southern border of Labrador westward : and that 

 such is the direction of the rivers, which farther west run on the 

 north into Hudson Bay, and on the south into the basin of the St. 

 Lawrence and the great western lakes, as to show that the elevated 

 land if not mountain ridges, in which these rivers take their rise, have 

 a direction nearly east and west. The same is true of the region west 

 of lake Winnipeck, and of a tract several hundred miles long, on the 

 south side of lake Superior, in which the waters that flow into that 

 lake take their rise. 



6. Northwest and Southeast System. 



No part of the region embraced by the geological map (Plate I.) 

 has occasioned me so much perplexity to ascertain the direction 

 of the strata, as that in the vicinity of Blackstone river, near the 

 place where it passes into Rhode Island. The direction of the 

 strata of gneiss there, does not generally differ much from the usual 

 course of the river, and the pretty uniform dip is northeast. Yet 

 sometimes we find them running nearly east and west and dipping 

 north : and this circumstance has led me to enquire, whether they 

 might not in fact be connected with the east and west system. I know 

 not that it is yet settled, how much local causes may alter the direc- 

 tion of strata of the same system in different places. But my present 

 impression is, that the strata under consideration can hardly be con- 

 nected with the east and west system: and, therefore, I place them 



