THE METAMORPHISM OF GLACIAL DEPOSITS A477 
planes there has taken place either a secondary alteration or a depo- 
sition from percolating water (Fig. 2). In some cases this secondary 
deposit has weathered away more rapidly than the wall material; 
in others, less rapidly. 
4. Weathering—Leaching in a relatively short time removes 
carbonates, especially from surface deposits. Only at a considerable - 
distance from the top do we often get evidence of carbonates in the 
superjacent drift. This leaching by ground water is the first step 
in the cementation process always going on at lower horizons. The 
bluish compressed drift invariably shows the presence of calcium 
carbonate. This fact does not imply that the drift had never lost 
its carbonates through leaching; it means only that now this particular 
cement is present, deposited probably from solution. No further 
observation was made to determine the cements or other chemical 
content of this dense drift. It is tentatively assumed that the uni- 
versal bluish color is a result of alteration, though it cannot be dis- 
proved that this drift in both New York and Ohio was not bluish 
from the time it was deposited, but the force of this possibility is 
somewhat lessened by the fact that there is considerable difference 
in the content of the drift of these areas; it is assumed, further, that 
this color probably represents a chemical alteration accompanying 
metamorphism, a change brought about, under particular condi- 
tions, by ground water in unconsolidated sediments. The nature 
of these conditions will be discussed later. 
The superjacent yellow till usually shows the results of weather- 
ing, especially near the surface; but in all parts there is evidence of 
leaching. 
AGENCIES OF ALTERATION 
Normally most of the changes going on in the regolith are due to 
pressure and to chemical reactions. ‘The pressure is that of the super- 
incumbent mass which varies directly with the depth. Chemical 
reactions are chiefly associated with water which is always a solvent, 
but the water of glacial drainage, since it comes in contact with such 
a wide range of rocks, is highly solvent and has capacity for other 
chemical reactions. 
Chemical.—Outside of arid regions, sediments contain a good deal 
