ALKALI SPOTS OF THE YOUNGER DRIFT SHEETS 263 



heretofore in use are apparently not sufficiently delicate to differ- 

 entiate such recent glacial deposits. 



The writer's personal observations are perhaps much too limited 

 to warrant the assertion that a criterion of the required delicacy 

 has been obtained, but there will be no harm in expressing the opinion 

 that more extended observations will probably confirm the pre- 

 liminary indications. There are abundant reasons for assuming 

 a priori that in the earliest stage of the after history of a drift- sheet 

 it will be characterized by a noticeable amount of sulphates and that 

 these will begin to disappear long before any appreciable diminution 

 of calcium carbonate or decomposition of ferro-magnesium minerals 

 have occurred — granted, of course, that the drift contains pyritiferous 

 minerals, which in fact is always the case. Pyrite is peculiarly 

 susceptible to the action of moist oxygen and its conversion into 

 ferrous sulphate and free sulphuric acid follows its exposure to the 

 dissolved oxygen of the ground water. Considering the relatively 

 small amounts of pyrite and sulphates in the drift as compared with 

 calcium carbonate, the great excess of atmospheric oxygen over 

 atmospheric carbon dioxide, and the great solubility of the sulphates 

 as compared with the solubility of calcium bicarbonate, it is quite 

 natural to expect that the surface of the till will be desulphurized 

 much sooner than it will be decarbonated. 



