588 THE GLACIAL LAKE AGASSIZ. 



in sjn'ings, as just noted and as observed in the channels of Two Rivers and 

 other streams, there are noticed also occasional and rare patches of ground, 

 usually no more than a few square rods, or at most a few acres, in extent, 

 on which wheat, oats, or other crops, after gei'minating- and beginning an 

 apparently health}' growth, are soon dwarfed or killed, while closely adjoin- 

 ing land, sometimes scarcely distinguishable in the appearance of the soil, 

 is yet divided from the preceding by rather definite boundaries, as shown 

 by the healthful growth of vegetation and a satisfactory harvest. These 

 peculiar spots fail year after year to produce any crop, and their exceptional 

 character in fields which mainly have a bountiful growth of waving grain 

 is a source of wonder and much conjecture to many farmers. They are 

 very simply explainable, however, as the places to which the saline and 

 alkaline artesian and spring water percolates upward through veins and 

 layers of gravel and sand and somewhat porous or creviced tracts of the 

 till and lacustrine and alluvial beds, until it comes to the surface and is 

 there diffused through the soil of a small or somewhat large area, thus 

 affecting vegetation, though not issuing in sufficient cpiantity to produce 

 springs. 



Although the bowlders of the till within the basin of the Red Riv^er 

 are mostly Archean granite, gneiss, and crystalline schists, derived from the 

 northeast and north, with few — pi'obably on an average less than 1 per 

 cent — of magnesian limestone, derived from Silurian and Devonian forma- 

 tions underlying the di-ift and outcropping northward in Manitoba, the latter 

 forms a very considerable proportion, usually more than half, of the smaller 

 rock fragments inclosed in the till and of the gravel in modified drift and 

 alluvial deposits. Owing to the greater prevalence of joints in the lime- 

 stone, it has been reduced more readily to the size of gravel, and it probably 

 has contributed at least as much as the Archean rocks to the sandy and 

 clayey matrix of the till, ptilverized by the g-rinding action of the ice-sheet. 

 The powdered limestone is one of the most important ingredients of the 

 drift. Dissolved in the water of wells and springs, as noted in the preceding 

 chapter, it makes them hard, diminishing their desirability for washing and 

 for use in the boilers of steam engines, but not for drinking and cooking-. 

 On the other hand, this element contributes a large share toward making 



