Geology of Massachusetts. 33 
Graywacke. 
For the most part, this rock furnishes a coarse stone only fitted for a 
common wall; but sometimes its stratification is so regular, and its 
grains are so fine, that it answers well for underpinning, step stones, 
&c. Itis quarried I believe in Brighton, and some other towns in the 
vicinity of Boston. At Pawtucket, on the R. I. side of the river, is 
an extensive quarry of a fine grained and slaty variety, which I should 
judge would form a good flagging stone; and immense quantities have 
been taken away for this object and for other purposes. On Canon- 
icut island in that state, is also a valuable quarry of this rock. 
Graywacke is sometimes beautifully amygdaloidal: that is, it con- 
tains numerous rounded or almond shaped nodules of some.other 
mineral. In these instance, however, the base of the rock is rather 
Wacke, than graywacke. This wacke (which resembles indurated 
clay,) often forms the cement of graywacke. In Brighton it is of a red- 
dish color, while the imbedded nodules are sometimes white, and 
sometimes white feldspar with epidote, which is of a lively green 
color; and these substances are not only in rounded masses, but in 
veins of irregular shape. ‘The rock is hard and may be even pol- 
ished. It then resembles porphyry and is very elegant. A fine 
example of this may be seen at the residence of H. A. S. Dearborn, 
Esq. in Roxbury, forming a pedestal for the bust of his father. It is 
only slightly polished, but would generally be mistaken for porphyry. 
A similar amygdaloid occurs in Brookline, Newton, and Need- 
ham. A variety still more beautiful is found at Hingham. The col- 
or of the base is chocolate red; and the nodules are red, green and 
white. I do not know whether large blocks can be got out. 
I think upon the whole, however, that the finest amygdaloid oc- 
curs in Saugus, on the hill a few rods east of the meeting house. 
The base is a pleasant green, and the nodules white, compact feldspar, 
generally spherical, and thickly interspersed. If polished, it must be 
exceedingly fine; and I have little doubt that large blocks ean be 
obtained at this locality. 
Argillaceous Slate. 
A more common name for this rock, atleast for the most useful vari- 
ety of it, is roofslate: because it is used for forming the roofs of houses. 
I have been inclined sometimes to regard the ranges in Quincy, Wa- 
tertown, Charlestown, and Chelsea, as a fined grained variety of 
Vou. XXII.—No. 1. 5 
