540 T. L. WATSON LITHOLOGICAL CHARACTERS OF VA. GRANITES 



(1) The visible joints that lie entirely or approximately in the hori- 

 zontal plane are a surface feature, becoming less emphasized with depth 

 and are not observed in the lower portion of the rock in some of the deeper 

 quarries. As a rule, the planes separate the rock into thinner sheets at or 

 near the surface and into thicker sheets on depth. 



(2) The joints of this set are developed parallel to the granite surface. 

 In the flat surface exposures of granite this set of joints lie entirely in 

 the horizontal plane. In the gently arched exposures the joints observe 

 approximately the same degree of curvature as that of the granite surface, 

 and in the steep domes the joints are correspondingly steep, observing 

 parallelism with the doming surface. 



In the opinion of the writer, the above facts indicate weathering as the 

 cause of this set of joints; exfoliation to be largely attributed to temper- 

 ature changes.* Curved joints undoubtedly exist in some granite masses 

 below the exfoliating surface and, as Merrill says, are "the result of 

 torsional strains and once existing are lines of weakness which become 

 more and more pronounced as weathering progresses."! 



Doming granite masses do not occur in the Virginia area and the joints 

 belonging to the above class are practically horizontal. Plates 

 69, 70, and 72, reproduced from photographs taken in the Virginia quar- 

 ries, show a strong development of horizontal jointing. 



*G. P. Merrill : Rocks, rock weathering and soils, 1897, pp. 180-184. 

 flbid., p. 245. 



