256 Scientific Geology. 



As we pass from the range of porphyry and compact feldspar on 

 the south of Boston towards the graywacke, and if I mistake not at 

 the junction of the two rocks, (e. g. in Dorchester and Canton,) wo 

 meet with a rock of a peculiar character, whose origin appears to be 

 in part mechanical. The compact feldspar seems to have suffered 

 some degree of abrasion after its consolidation, and the fragments to 

 have been reconsolidated into a rock more or less slaty, with the ad- 

 mixture of but few foreign ingredients. (Nos. 301, 302.) It would 

 seem to have been partially fused the second time ; or perhaps it 

 might have been produced by the partial cooling of the compact feld- 

 spar at its junction with the graywacke, as it was forced through that 

 rock while in a melted state. This would account for its semi-brec- 

 ciated aspect and slaty structure, and the occasional presence of for- 

 eign ingredients. But this rock deserves a more careful examination 

 than I have been able to give it. 



Our compact feldspar is slaty in some other places ; as at New- 

 bury : but it does not appear to have been recomposed. 



3. Quartz Rock. I thus denominate two or three most singular 

 varieties of rock in the formation under consideration, because quartz 

 is their predominant ingredient. 



The most remarkable of these varieties is developed very distinctly 

 at the southeastern extremity of Rhode Island ; as may be seen by 

 the sketch already given of that portion of the Island. It consists of 

 coarse grains of hyaline quartz, of a purple color, passing to deep 

 blue and black, with tale or mica ; (it is difficult to say which ;) the 

 materials having a schistose arrangement. (Nos. 303 to 306.) The 

 quartz bears a strong resemblance to peliom, and constitutes a large 

 part of the rock. The aggregate exhales an argillaceous odor when 

 breathed upon. 



This same rock may be seen at the mouth of Fall River, in Troy, 

 where it is associated with an argillaceous slate, passing into mica 

 slate, and of a quite dark color from the carbonaceous matter it con- 

 tains. At this place, this slate and quartz rock are contiguous to gran- 

 ite ; and they may be seen in Tiverton, lying directly upon the granite. 

 In Newport, also, granite cannot be far distant from the same rock. 

 Do not these facts furnish a clue to the origin of the dark color of 

 the quartz ? Was it not penetrated by the carbonaceous matter of the 

 black slate, while in a state of partial fusion by the action of the 

 melted granite ? 



There can be no doubt but the quartzose rock above described, is 



