Ranges of Porphyry. 



447 



a very common or abundant variety : but for ornamental purposes it 

 affords specimens of great delicacy. (Nos. 1263 to 1270.) 



Topography of Porphyry. 



Only three ranges of this rock are given on the map; and these 

 are all in the eastern part of the state. Two of them, — the princi- 

 pal ones, — lie the one on the north and the other on the south of Bos- 

 ton, having their longitudinal direction east and west. The third is 

 in Essex county, extending easterly from Byfield Academy, nearly or 

 quite to the coast. This strip is chiefly compact feldspar and mostly 

 the red variety. It certainly deserves a more thorough examination 

 than I have been able to give it. 



In some places farther south, as for instance on the turnpike be- 

 tween Boston and Newburyport, in Topsfield, I observed a rock in 

 place, intermediate between porphyry and greenstone : and not im- 

 probably genuine porphyry may be found in the vicinity. 



This rock is most fully developed in its characters in the range a 

 a little north of Boston, extending from West Cambrige through Mai- 

 den and Medford to the east part of Lynn. The southern border of 

 this strip, certainly towards its westerly extremity, is compact feldspar. 

 The porphyry (mostly antique, though sometimes brecciated,) forms a 

 broken ridge of considerable height, generally naked and precipitous 

 on its southern side. On the north it is succeeded by sienite, and the 

 two rocks are so closely connected that the line between them is very 

 obscure and irregular. I am satisfied, however, that this porphyry 

 range has been usually represented too wide. This is the range that 

 will probably furnish the best varieties for ornamental purposes, when- 

 ever the public taste shall create a demand. 



There is reason to suppose that this range once extended much far- 

 ther east than at present. For Hon. H. A. S. Dearborn informs me 

 that Halfway Rock, lying in the ocean several miles east of Marble- 

 head, is porphyry. Indeed, specimen No. 1264, which he presented 

 to me, will show that it is the finest brecciated porphyry in the State, 

 which I have met with. Now as this island lies nearly in the direc- 

 tion of the Maiden and Lynn porphyry range continued easterly, I in- 

 fer that it did once reach so far ; (and perhaps does now beneath the 

 ocean ;) nor can we say how much farther. We see here from 

 whence proceeded the porphyry pebbles that are so common along 

 the southern shore of Massachusetts Bay, and on the islands of Nan- 

 tucket and Martha's Vineyard. 



