SOME GLACIAL AV ASH-PLAINS. 113 



veins of carbonate oi limo have invariably been robbed 

 of these veins by the downward percolation of rain-water 

 charged with acids from the soil and the air. Now and 

 then, the interior of a pebble exhibits a remnant of one 

 of these veins as a deliquescent lump of calcite marked 

 by spoon-shaped inosculating depressions, the character- 

 istic mark of solution. Deep clefts are freipiently opened 

 up along the cleavage planes of the calcite. The cavities 

 in many pebbles, thus formed by the removal of calcite, 

 constitute from a tenth to a dfth by volume of the rock. 

 Thousands of pebbles exhibit the same abstraction of car- 

 bonate of lime. 



Associated with but underlying this pebbly zone of 

 solution is oie in which the pei)bles exhiijit the redeposi- 

 tion of the carbonate of lime. This deposition of the 

 lime carbonate takes i)lace as in the case of stalactites in 

 caves, on the under side of the roof-like surface of the 

 larger pebbles which rest upon coarse sands below. A 

 crust of lime carbonate thus forms cementing the under- 

 lying sands to the overlying pebbles. On top of the peb- 

 bles which carry this lime crust is usually to be found a 

 film of dust, the mechanical load of the percolating water. 

 A few pebbles become encrusted over their entire surface 

 with carbonate of lime. 



This action is most noticeable in tlie northern or head 

 portion of the wash-plain, where the gravels are relatively 

 coarse. The lime carbonate layer is not more than tive or 

 six feet below the surface in some instances. It suggests 

 itself that the agricultural value of wash-plains might be 

 enhanced by penetrating to this lime-bearing zone and 

 returning the carbonate of lime to the soil by accumu- 

 lating heaps of the gravels from which the lime would 

 slowly, by the action of the rains, work its way into the 

 surrounding top soil. After such gravels have been 



