EFFECT OF LEACHING ON DRIFT PEBBLES 565 
the first column are the averages of thirty-seven samples of 100 
pebbles each, all from till which had not been perceptibly altered. 
In the second column are the averages of seven samples of 100 
pebbles each, taken from somewhat leached till. The third column 
represents averages of seven similar samples from till somewhat 
more affected by leaching. The fourth column shows the average 
percentages for seven s'milar samples taken from till representing 
extreme conditions of leaching. 
TABLE SHOWING CHANGES CAUSED BY LEACHING, IN PERCENTAGES 
OF DIFFERENT KINDS OF PEBBLES PRESENT IN BOWLDER CLAY 
Kinds of Rocks I 2 3 4 
Flint, chert, jasper (and felsite)......... ri 27.2 45.1 59.0 
AWFEIR CIAL EZ et ceet te eee eid are Che cr sio'e cei. s\ernis Fike Tag 173 21.0 
Sandstone and quartzite. ...........-:. 4.7 8.0 7.0 8.0 
Woneretionary, hematite. a. ....-.... >. rare) 3.0 1.8 1.0 
Graniteand pneisss. 22). 2. ek oe ec oe ns TAU 11.8 he Goi 
Greenstone, schists (and shale)......... sas 70 S15) 1.9 
Diabase and other volcanic rocks........ 10.5 igs & 8.6 2.6 
By glonatiG WMEStONE fe V6.) sre we ss se aes 20.10 5-3 2.4 0.2 
(Calcareousilimestone..2...-..--...--- 27.0 Be 2 0.5 0.0 
Decayed limestone and undetermined 
ROCKS EAE See aisee Saree: clk) ofc lbadls’ Se livsls Ons 5-4 BO 16 7/ 
Under the action of the destructive agencies to which the 
pebbles of the drift have been exposed in this region, the ratios of 
the numbers of pebbles of different classes of rocks have undergone 
progressive changes. These changes indicate that the groups of 
samples represent successive stages of a general progress of solution. 
This progress involved a gradual removal of the most soluble rocks. 
The different kinds of materials are arranged in the table in the 
order of decreasing resistance to destruction. The end of the 
process in this case is the removal of practically all pebbles consist- 
ing of calcareous limestone. This results in a corresponding 
increase in the ratios of the numbers of the most resistant pebbles. 
This increase is greater or less according to the relative resistance 
of each class of rock. 
Beginning with flint, we find that while these pebbles count 
only about eleven to a hundred in the till that still remains un- 
changed, they make, in the most thoroughly leached till, 59 per 
