Mctauiorp/iism of Glacial Deposits 5 



cinm 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 universal bluish color is a result of alteration, though it cannot 

 be disproved that this drift in both New York and Ohio was not 

 bluish from the time it was deposited, but the force of this possi- 

 bility 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 par- 

 ticular conditions, by ground water in unconsolidated sediments. 

 The nature of these conditions will be discussed later. 



The superjacent yellow till usually shows the results of 

 weathering, 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 superincumbent 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 con- 

 tact 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 of water. In all climates circulating ground water exists at 

 some depth; the more humid the climate, the higher is the ground- 

 water level. It is probable, however, that a special condition exists 

 in sediments subjacent to an ice-cap; here, on account of the con- 

 stant melting of the basal ice caused by radiation from the earth,* 

 the supply of water is so great that a condition of saturation exists 

 in these sediments. This condition of saturation was certainly the 

 case during both the advance and retreat of the ice-sheet within the 

 north-sloping side of the St. Lawrence drainage basin. This north- 

 ward slope in conjunction with th.e wall of ice caused a ponded con- 

 dition of drainage. Beneath these bordering lakes, sediments were 

 alwavs in a condition of saturation. 



