296 THOMAS L. WATSON 



from hand specimens of the rock collected. Certain peculiarities of 

 occurrence, structure, and composition of this rock not previously 

 noted, so far as I am aware, among deep-seated rocks developing the 

 spheroidal structure, seemed worthy of careful study. Further interest 

 attaches to this rock for the reason that, as yet, it forms the only 

 example of orbicular structure among deep-seated rocks found in 

 the southern Appalachian region, and also because it adds a rock 

 type which, in some respects, is a new one developing nodular or 

 spheroidal structure. 



GENERAL DESCRIPTION OF THE OCCURRENCE. 



Exposures of the orbicular rock occur in the eastern part of Davie 

 county on the Hairston farm, about ten miles west of Lexington and 

 within one and a half miles of the Yadkin River. A peak or knoll 

 of moderate size, rising about thirty feet in elevation above the sur- 

 rounding plain and composed of huge bowlders, affords the only 

 exposure of the fresh rock. Several of the larger bowlders have been 

 split and partially worked off at different times, chiefly for use about 

 the Hairston residence and to a less extent for museum specimens. 



Traced southwestward from the knoll is found complete evidence 

 of the extension in that direction of the orbicular gabbro-diorite, in 

 the residual decay and in occasional partially decayed fragments of 

 the rock scattered over the surface. The decay is of a pronounced 

 dark, nearly black color, with a distinct greenish tint imparted by the 

 ferromagnesian constituent of the fresh rock. Oxidation of the 

 iron in the iron-bearing minerals of the decay is not appreciably 

 apparent at any point. As nearly as could be determined, the zone 

 of residual decay derived from the gabbro-diorite averages several 

 hundred feet in width and extends approximately one-half to three- 

 quarters of a mile southwest from the koll. Fairly sharp contacts 

 between the decay of the orbicular diorite and that of an inclosing 

 gray porphyritic biotite granite were noted in a number of places, 

 which strongly suggest that the orbicular rock forms a dike having 

 an approximate northeast-southwest strike, penetrating the porphy- 

 ritic biotite granite. 



is made in the following publications: J. V. Lewis, "Notes on Building and Orna- 

 mental Stone," First Biennial Report of the State Geologist, 1891-92, North Carolina 

 Geological Survey, 1893, p. 91; George P. Merrill, Stones for Building and Decora- 

 tion (New York, 1897), 2d ed., p. 259. 



