448 



Scientific Geology. 



The porphyry range south of Boston occupies much more of the 

 surface than that just described ; and yet I doubt whether it contains 

 so much genuine porphyry. It extends with some apparent inter- 

 ruptions, (though I doubt whether there are any real interruptions,) 

 from Natick to Hingham, through a part of Needham, Dedham, Mil- 

 ton, Braintree, Quincy, and Weymouth. I have not found it, how- 

 ever, in the latter place, and have marked the deposit of this rock in 

 Hingham as insulated from the rest. This patch is chiefly compact 

 feldspar. The range it will be seen embraces the greater part of the 

 Blue Hills, the most elevated land in the vicinity of Boston, its high- 

 est point rising more than 700 feet above the ocean. But it is only 

 the upper part of this mountain that is composed of porphyry, and 

 by no means the whole of its summit neither : for sienite is frequent- 

 ly found there. The porphyry is chiefly that variety which has a 

 trachytic aspect, being evidently intermediate between porphyry and 

 sienite. / 



I have met with rocks approaching to porphyry in no other places 

 in the State, except one or two. In the sienite of Whately I found a 

 vein of compact feldspar two or three feet wide; but the foliated 

 structure of the feldspar was not entirely obliterated. In the argilla- 

 ceous slate of Guilford, Vt, a quite distinct porphyroid granite oc- 

 curs, and with it well characterised greenish compact feldspar. These 

 rocks are so obviously granite, imperfectly melted down, that I have 

 thought it best to describe them under granite, and to place the spe- 

 cimens (Nos. 1467 to 1470,) among those of granite. A speci- 

 men (No. 1211,) of the Whately compact feldspar will be found in 

 the collection. 



Geological Position. 



Of all the questions that have exercised the ingenuity of geologists, 

 none appear to me more perplexing and unprofitable than those which 

 they have raised and discussed relative to the primitive, transition, 

 and secondary character of porphyries. In reading Humboldt's re- 

 marks on the transition porphyries, in his Essay on the Superposi- 

 tion of Rocks, I have been reminded of a man benighted in a quag- 

 mire. Every effort which he makes to extricate himself, only plunges 

 him in deeper. Am I asked whether the porphyry of Massachusetts 

 belongs to the Primitive, Transition, or Secondary Class ? I reply - 

 that it belongs to none of them, but is a member of a series of rocks 

 consisting of granite, sienite, porphyry, and greenstone, which have 



