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 Graywacke 

 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- 

 ley 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 



