Porphyry. 



21 



and enduring a rock. Whenever this shall happen, the vicinity of 

 Boston will furnish every variety than can be desired, and in blocks 

 large enough for any purpose. Quite a number of smoothed or pol- 

 ished specimens may be seen in the collection. 



The porphyry range on the north of Boston, is most perfect in its 

 characters, and in the greatest abundance at any one place ; although 

 the southern range spreads over a greater extent of surface. In 

 Lynn, and some other towns, I have observed blocks of porphyry 

 that were brecciated — that is, they were composed of angular frag- 

 ments of porphyry reunited. This furnishes a beautiful variety for 

 polishing. 



Sienitic Porphyry. 



When sienite contains crystals of feldspar imbedded in the mass, it 

 is said to be porphyritic ; and some varieties of this rock in the eastern 

 part of the State are very elegant. Essex County produces some of 

 the finest specimens, particularly Cape Ann. Sometimes the imbed- 

 ded crystals of feldspar, are white, sometimes flesh-colored, and in 

 Gloucester, I found a rock in which they were of a rich bronze color. 

 These sienitic porphyries are extremely elegant when polished ; but 

 I am not aware that they are employed at all for ornamental purposes, 

 in this country. 



Porphyritic Greenstone, 



The ingredients of greenstone are often not easily distinguished 

 from each other by the naked eye ; and when, in such a case, the 

 rock contains disseminated crystals of feldspar, it becomes porphy- 

 ritic. If these crystals are greenish white, and the base blackish 

 green, the rock is the green porphyry of the ancients. In Dorches- 

 ter, Brookline and Roxbury, according to the Messrs. Danas, it oc- 

 curs in rounded masses ; and in small quantity, in veins, at Marble- 

 head. But I have found it in large veins, traversing sienite, at Sandy 

 Bay, on the north-east side of Cape Ann. Large blocks might be 

 thence obtained ; and if polished, it would constitute a truly splendid 

 ornament for the interior of a church, or a private dwelling. 



If the feldspar crystals be black, or greyish black, the rock is the 

 superb black porphyry of the ancients. This occurs in small beds and 

 rolled masses in Charlestown, and in veins of greenstone, at Marble? 

 head, according to the Messrs. Danas : but I have not met with it. 



