20 Geology of Massachusetts. 
gneiss, for instance, is very much used in Worcester, and does not 
there present any appearance of stratification, and very little of a 
slaty structure: while the granite, that is quarried in the east part of 
Worcester, is distinctly stratified ; and would probably be called 
gneiss, by most persons, rather than the Millbury rock. The Worces- 
ter granite, however, affords almost the only example of stratification 
in that rock in this State. 
At these gneiss quarries it is easy to obtain blocks from ten to 
twenty feet long, which are only a few inches thick. At Dudley, I 
was told that narrow slabs of this rock, such as would answer for 
posts, or side walks, could be split out, aud delivered in the center of 
the town, for four cents per foot. 
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 
limestone. 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. 
Fornblende Slate. 
I do not recollect to have seen this rock employed in Massachu- 
setts for any useful purpose, except for the construttion of common 
stone walls. But { have noticed some very fine samples of it in the 
flagging of the side walks of New Haven, obtamed, I presume, in 
Connecticut, from the same range that passes through Monson, Ware, 
Xe. in Massachusetts. 
Porphyry. 
This term, as it is employed in the arts, embraces several varie- 
ties of rock not designated by its strict geological sense. Although 
upon the Map, I have included in the term, ouly the porphyry of ge- 
clogists, 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 
