34 Economical Geology. 



large blocks can be obtained at this locality ; but as the base is softer 

 than the nodules it can be only imperfectly polished. 



Argillaceous Slate. 



A more common name for this rock, at least for the most useful 

 variety of it, is roof slate; because it is used for forming the roofs of houses. 

 I have been inclined sometimes to regard the ranges in Gluincy, Wa- 

 tertown, Charlestown, and Chelsea, as a fined grained variety of 

 graywacke ; but this question may be more properly considered in 

 the scientific part of my report. At any rate, this rock, in the towns 

 above mentioned, does not split into layers sufficiently thin for roof- 

 ing. But it is valuable for grave stones, the covering of drains, flag- 

 ing stones, &c ; and for these purposes it is extensively wrou ght in 

 Q,uincy r Charlestown, &c. 



Novaculite. 



This is a variety of argillaceous slate which is known in the arts 

 under the name of hone, oil stone, turkey stone, and whet stone. It 

 is in beds of argillaceous slate in Charlestown, Maiden and Q,uincy. 

 It is not however, of a very good quality ; and I am not aware of its 

 being used for hones, or even for whet stones ; although it might an- 

 swer the purpose, if better materials could not be found elsewhere. 

 Mr. Godon in his account of the geology of Boston and vicinity, says 

 that a compact feldspar is found there perfectly analagous to the Tur- 

 key stone. I have found a variety of this mineral in Newbury, which 

 I apprehend, corresponds with that described by this writer, and a spe- 

 cimen may be seen in the collection ; but no fair trial that I know of 

 has been made to employ this stone as a hone. 



Roof Slate in Worcester County. 



The range of slate exhibited on the Map in the towns of Boyls- 

 ton, Lancaster, Harvard, Shirley and Pepperell, is associated with 

 the peculiar mica-slate that contains the Worcester coal. It answers 

 for roofing in some parts of the bed and has been quarried for this pur- 

 pose in Lancaster. It has been wrought considerably in Harvard 

 and Pepperell for gravestones ; and is transported a considerable 

 distance for this purpose. The stratum is narrower near the north 

 line of the state ; but I have found no time to ascertain how far it ex- 

 tends into New Hampshire. 



