ARGILLACEOUS SLATE. 231 
tained if we succeed in pointing out the relative sit- 
uation of those rocks usually assigned to this class, 
and their topography in this country. 
These rocks are, 1. Clay Slate; 2. Transition or 
Mountain Limestone ; 3. Graywacke and Graywache 
Slate,, passing into Old Red Sandstone, 
Argillaceous or Clay Slate. — We have already re- 
marked that this rock is composed of clay, more or 
less indurated, and readily splits into distinct lam- 
inae ; from which circumstance it is called roof-slate. 
Some geologists make this rock include the gray- 
wacke formation; others exclude the latter. This 
rock abounds in the vicinity of mica slate, into 
which it seems to pass by insensible gradations. 
Argillaceous slate forms three ranges in Massachu- 
setts, viz., in the counties of Worcester, Franklin, 
and Berkshire ; extending up the Connecticut Val- 
l-ey far into New-Hampshire and Maine, and through 
the whole extent of the western boundary of the 
state far into Vermont, and probably to Canada. 
It forms a great part of Columbia and Rensselaer 
counties, and is wrought for roofing purposes in 
Hoosack, Lebanon, and Hillsdale. It is connected 
on the east with mica slate ; on the west with gray- 
wacke, passing into both. Near the Susquehanna, 
in Pennsylvania, it is extensively quarried for roofs, 
and over 1600 tons annually procured, which sells 
in Baltimore at 22 dollars a ton. A ton of slate 
forms about 200 superficial feet when on the roof. 
It is unnecessary to specify the topographical dis- 
tribution of this rock ; suffice it to say that it accom- 
panies the primary rocks throughout the United 
States, extending from Canada, in patches, or a 
continuous formation, to Alabama. The strata of 
this rock, for the most part, dip to the east with an 
angle of from 15 to 90 degrees. Shale is only a va- 
riety of argillaceous slate, containing a greater pro- 
portion of carbon, but it is often a later formation. 
These rocks belong to the graywacke and lower 
