20 Economical Geology. 



Greenstone. 



This is one of the most enduring of all rocks ; but it is usually so 

 much divided by irregular seams, into small and shapeless blocks 

 that it is but little employed, either in the construction of houses, or 

 walls. Its dark color, also, renders it less acceptable than granite or 

 limestome. Still it is beginning to be used for building houses, in its 

 unaltered state. The irregular blocks may be so laid with white mor- 

 tar, especially in the Gothic style of building, as to form a picturesque 

 and pleasing structure. The Episcopal Church, in the city of New- 

 Haven, (Conn.) presents a good example of this kind of architecture. 



Hornblend Slate. 



I do not recollect to have seen this rock employed in Massachu- 

 setts for any useful purpose, except for the construction of common 

 stone walls. But I have noticed some very fine samples of it in the 

 flagging of the side walks of New Haven, obtained, I presume, in 

 Connecticut, from the same range that passes through Monson, Ware, 

 &c. in Massachusetts. 



Porphyry. 



This term, as it is employed in the arts, embraces several varieties 

 of rock not designated by its strict geological sense. Although upon 

 the Map, I have included in the term, only the porphyry of ge- 

 ologists, yet in this place, I shall describe all those compounds oc- 

 curring among us, which have been denominated porphyry in the arts. 



The first and most extensive of these, is the genuine feldspar por- 

 phyry, represented on the Map in large quantities in the towns of 

 Medford, Maiden, Chelsea and Lynn, on the north of Boston ; and in 

 Needham, Milton and Braintree, on the south. This is the oldest 

 and most enduring of the porphyries, and, indeed, the hardest of the 

 rocks. Its basis is generally compact feldspar, reduced to a homo- 

 geneous paste, and of various colors; as light purple, red of various 

 shades, brownish black, and greenish gray. The imbedded crystals 

 are either feldspar, or quartz, alone, or existing together in the same 

 rock ; and their colors are very various, though more usually white 

 or gray. By these mixtures porphyries are produced, rivalling in 

 beauty the best antique porphyry. This rock is polished with so 

 great difficulty, that it is rarely used in our country, either for orna- 

 mental or useful purposes. But it would be strange if an increase of 

 and refinement should not create some demand for so elegant 



