Geology of North Carolina. 243 



=at right angles to their sides, form the sides of the wall ; and 

 since the wall dips eastward, at an angle 78 1-2 degrees, the indi- 

 vidual columns of course dip westward at angle of 11 1-2. 



Course of the wall (as near as we could judge without a com- 

 pass) south thirty degrees east. 



This dyke though smaller than that at Robley's, on the south 

 Yadkin, (which is the one generally intended by the natural 

 wall) is still more regular than that, and in every respect more 

 interesting. The breadth being so small, and the prismatic col- 

 umns lying so closely compacted, we found it easy to remove 

 entire sections of the wall, several of which I had put up in 

 boxes for the inspection of geologists abroad. Robley's wall 

 however, has been extensively visited and is minutely described 

 in several publications. The description of it given by Dr. 

 Beckwith, in the 5th volume of the American Journal of Science, 

 I found to be so accurate as not to require any further obser- 

 vations from me. 



The mica slate of the granitic district is interesting on ac- 

 count of some valuable imbedded minerals. Among these 

 are extensive beds and veins of iron ore, a general account 

 of which we have already given. Here occur also beds of 

 white primary limestone, with a coarse grain. It is in mica 

 slate also, that the very extensive deposit of North Carolina 

 plumbago exists. We see not why this bed will not become 

 of great value when extensively wrought. An account of it 

 is thus given in the Report. 



Plumbago of Wake. 



Locality and extent. — This great deposit of Black Lead lies a 

 little westward of Raleigh, and is crossed by all the roads thai 

 lead to Hillsborough. On the road to Chapel-Hill, I have ob- 

 served it in a gulley within two and a half miles of the capitol, 

 which is, I believe, its nearest distance from Raleigh. On the 

 same road it is seen again a little west of Mrs. Streeter's ; and 

 a short distance north of this place, on the Hillsborough road, is 

 one of the largest beds that I have seen any where expo- 

 sed to the surface. Its apparent width is about twenty feet. 

 On the eastern road to Hillsborough, we fall in with the forma- 

 tion soon after passing Crabtree creek, three and a half miles 

 from Raleigh. At this place is Guthrie's mine, where the prin- 

 cipal excavations have been made, and where has been obtained 

 the greater part hitherto exported. 



The whole formation consists of a great number of parallel 

 beds varying in width from a few inches to twenty feet. They 

 lie in a singular variety of isinglass rock (micaceous shistus) usu- 



