Weathering of Diabase in Virginia, — Watson. 355 
SOME FURTHER NOTES ON THE WEATHERING 
OF DIABASE IN THE VICINITY 
OF CHATHAM, VIRGINIA.* 
By Thomas L. Watson, Atlanta, Georgia. 
Statement. — In a former number of this journal, f the writ- 
er pubHshed the resuhs obtained from a careful study of the 
w-eathering — transition from fresh to decomposed, incoherent 
iQel^ — of one, of the Mesozoic diabase dikes, near Chatham, 
Virginia. The data indicated that in passing from the fresh 
to decomposed rock, the transition was accompanied by an 
unusually large loss in the alumina,;]: which has, hitherto, been 
considered one of the most refractory constituents , by investi- 
gators working along similar lines — serving in many cases as 
the constant for calculating the loss and gain in the other 
constituents. 
No explanation was offered at the time, as the work was 
considered too local, and not extensive enough, to warrant 
any statement to account for this condition. Soon afterwards, 
however, the writer was given the opportunity to collect and 
examine a similar series of specimens from an adjacent dike, 
for the purpose of covering the same ground as in the former 
paper, with the view of seeking a satisfactory explanation for 
the loss in alumina. 
Location and description of rock\ The dike is cut across 
l)y the Southern railroad one mile west of Chatham, with an 
exposure in width at this point of something over 150 feet. 
The railroad cut is about 25 feet deep, consists of large and 
small boulders piled on one another, which become smaller 
and almost completely decomposed near the top of cut, where 
they are wholly embedded in a light yellow stained, mottled 
clay — a product of residual decay from the diabase rock. The 
limit of decay — weathering— is, in other words, considerably 
below the depth of cut. 
Megascopically, the rock is dark gray in color, and uni- 
*The writer is indebted to Prof. Geo. P. Merrill, Head Curator, 
Dept. of Geology, U. S. National Museum, for kindly reading and 
commenting on the paper in MS. form. 
tWatson. Thos. L.. Weathering of Diabase near Chatham, Virginia, 
Am. Geol. 1898, 22, 85-101. 
$Il)id. 
