154 



Scientific Geology. 



eastern portion of Massachusetts, without having the enquiry forced 

 repeatedly upon his attention, whether the gray wacke formation, 

 that now exists only in interrupted basins from Rhode Island to New 

 Hampshire, was not once continuous throughout this whole extent. 

 The direction and dip of its strata, as well as its mineralogical charac- 

 ters prove it to have been mostly produced at the same geological epoch; 

 and the very powerful marks of degradation, which some of its vari- 

 eties exhibit, especially the conglomerate, favor the opinion of its for- 

 mer continuity. And yet no one will presume to impute such pow- 

 erful abrasion to any single deluge. 



The particular towns, where we find the most striking examples 

 of a mixture of bowlders of granite, sienite, and greenstone, with 

 those of gray wacke, which is the rock in place, are Attleborough, 

 Mansfield, Norton, Bridgewater, Brighton, Newton, Needham, and 

 Watertown. The map will show, that a few miles northerly from 

 these towns, are deposites of granite, sienite, and greenstone. On 

 the other hand, in Stoughton, Randolph, Dover, Dedham, Brain- 

 tree, &c. we find gray wacke bowlders, mixed with those of the rocks 

 in place ; and these were obviously derived from the gray wacke 

 formations lying northerly from these places. 



Perhaps the example more definite and decisive than any other on 

 the subject under consideration, occurs in Rhode Island. In Cum- 

 berland a large hill exists of magnetic iron ore ; a considerable part 

 of which contains distinct crystals of feldspar, so as to become beau- 

 tifully porphyritic. A rock so peculiar cannot be confounded with 

 any other. Now if we pass along the north, east, and west sides 

 of this bed of ore, even very near it, no scattered fragments of it 

 are seen among the bowlders. But on the south side, they occur 

 all the way to Providence, decreasing in size. Whether they may 

 be found on the west side of Narraganset Bay, south of Providence, 

 I cannot say : but I met with several pieces at the southern extremity 

 of Rhode Island, in Newport, of only a few inches in diameter. 

 These must have traveled nearly 35 miles from their bed, in a direc- 

 tion a few degrees east of south. 



In several places in the south east part of Worcester county, I met 

 with bowlders of a variety of porphyritic granite, distinguished from 

 every other kind in the State, by its remarkably large imbedded crys- 

 tals of white feldspar. But it was not till I came to Harvard, that I 

 found this rock in place. On the north of the ledge, I never met 

 with a single fragment. In Waltham, however, I did meet with one 

 bowlder of this rock. 



