EFFECT OF LEACHING ON DRIFT PEBBLES 567 
more gradually than does the leaching-away of limestone. In a 
partially leached till, where some limestone pebbles are still left, 
the ratios of these rocks to the total are higher than in the unaltered 
till. In the classes named, gneiss, schists, shale, and volcanic 
rocks, other than dark diabase or diorite, form unimportant parts of 
the groups. Basic crystalline rocks evidently resist leaching and 
weathering less effectually than the acidic rocks. 
Dolomitic limestone and calcareous limestone, which occur in 
about equal quantities in this drift, are both much more promptly 
removed than any of the other rocks. The ratio of such pebbles to 
the total is reduced to less than one fourth of its original value before 
the other rocks have been much affected. In the final stage the 
ratio of the dolomite pebbles to all others is 1: 500, and the calcar- 
eous limestone has disappeared entirely. A limestone pebble is 
extremely rare‘in till, the body of which gives no reaction for 
carbonate of lime. 
These observations show that under the conditions of weather- 
ing and leaching in the prevailing climate and drainage of this 
region, flint is one of the most enduring materials. It is the last 
thing to yield to solution and general weathering. It is relatively 
highly insoluble and tough. This explains why flint is a common 
and large ingredient in the gravel veneer found on the plains and 
plateaus of the West. It is always an important, and in places 
almost the only, ingredient in the oldest Pleistocene gravels of the 
South. The Uvalde formation in Texas consists of gravel consist- 
ing almost entirely of flint. About 70 per cent of the gravel taken 
in the Ohio at Cincinnati consists of flint and quartzite, brought 
largely from the residual surface material in the upper basin of this 
stream. Such gravels are also common in some parts of the Penn- 
sylvanian in the central states, and these sediments are known to 
have been deposited during an era of extensive erosion and hence 
also general leaching of the land. 
