500m © J. A. UDDEN 
cent. In one of the seven samples of too pebbles of which 59 per 
cent is an average, there were 70 flint pebbles. In the group of 
flint were counted a few pebbles of jasper and of a black felsite. 
Some, which here is called flint, should perhaps have been called 
chert, which contains some calcareous material. This is present as 
a mixture throughout the mass of the chert, and in the leached con- 
dition of the chert it has been dissolved away, leaving a rock which 
is a porous silica that readily absorbs water. 
Under “vein quartz” were classified the white quartz pebbles 
which are derived from quartz veins in crystalline rocks, and which 
themselves have a crystalline structure, although the external 
crystalline form is hardly ever developed and only very rarely pre- 
served in the pebbles. This form of quartz is somewhat less 
resistant than flint. While the percentage of flint is more than 
five times greater in the most leached till than in the unleached, the 
percentage of vein quartz is only three times as high. 
Sandstone and quartzite were classified together for the reason 
that in the most thoroughly leached till it is often impossible cor- 
rect y to distinguish the two. In leached quartzite the bond unit- 
ing the grains is sometimes weakened and the rock has in effect 
again become a sandstone. This may lead to the false conclusion 
that sandstone resists weathering better than does quartzite. 
Sandstone is, of course, a rock of variable qualities of resistance 
and strength. The varieties of sandstone and quartzite represented 
in this drift together are a little less enduring than vein quartz. 
It appears that they are only about twice as numerous as the vein 
quartz pebbles in the most thoroughly leached till. 
Angular pieces of hematite of variable hardness are present in a 
small quantity in this till. Sometimes they are in part carbonate 
of iron. They were originally fragments of clay-iron-stone con- 
cretions, which are now mostly changed to iron oxide. Observa- 
tions on compact hematite pebbles on the plains indicate that these 
are almost as enduring as flint, but the hematite pebbles in this till 
suffer somewhat more rapid destruction than sandstone and quartz- 
ite and are only about as enduring as granite and gneiss. 
At first, the destruction of hematite, granite, gneiss, greenstone, 
schists, shale, diabase, and other igneous rocks proceeds much 
