86 TJie American Geologist. August, i898 
Althono;'h the similarity in the above respects are verv 
close, the same rocks, according to locality, ofifer wide differ- 
ences in their final products of decay. These dififerences are 
not without their importance and significance, from an eco- 
nomic as well as a scientific standpoint. 
Locality. The material, from which the results given below 
were obtained, was collected from a series of dikes of con- 
siderable though variable widths, in the southern and central 
part of Pittsylvania county, Virginia, a few miles west of the 
county court house. Here the dikes break through the old 
crystalline schists and gneisses of the Piedmont plateau, but 
when traced for a short distance to the east and south they are 
found intersecting the Triassic shales and sandstones of the 
"Danville area."* 
The dikes are numerous throughout the county, and good 
exposures frequently occur, the best of which -are seen in and 
along the cuts of the Southern railway. Generally, they can 
be traced with considerable accuracy over the surface by their 
weathered products, mostly in the form of boulders, which, 
on account of their dark color and hardness, are called 
"niggerheads." The extreme depth in decay of both dike and 
country rock, renders, even in the deepest cuts, the differentia- 
tion of the contact between the two extremely difficult and 
often impossible. 
Field Character of the Rock. Texturally, the rocks here 
studied are of two kinds. The first is a very uniformly coarse 
granular rock of a dark gray color, with a decided greenish 
cast, and a more or less greasy lustre. In the fresh and un- 
weathered hand specimen, feldspar, augite and olivine can be 
detected with the unaided eye. 
The second is a more homogeneous fine grained rock, of a 
dark steel gray color, in which only feldspar and augite are 
recognizable with the naked eye. 
The final products of weathering are apparently the same 
for the two rocks, and therefore, cannot be differentiated from 
each other in the field. In each case, the residue is a tough 
clay of a bright red color. Weathering in concentric layers, 
so generally characteristic of basic eruptives, is beautifully 
*For a description and extent of this area, see Correlation Papers, 
Newark System. Bull. No. 8s, U. S. G. S., p. 22. 
