116 BULLETIN OF THE ESSEX INSTITUTE. 



tain exactly the amount of carbonate of lime which has 

 been removed. In those cases where some of the veinstone 

 still remains, from an extended series of observations there 

 might be determined the rate of solution and so the dura- 

 tion of the post-glacial interval.^ 



ECONOMICS OF WASH-PLAINS. 



The wash-plains of this region play a very important 

 role in the settlement of the country. Professor Shaler 

 has noted the choice which they offered, to the early settler, 

 of flat lands freed from the boulders encumbering the 

 till-covered uplands. Although the wash-plain soils are 

 sandy and relatively dry, the small amount of labor re- 

 quired to put some of the less elevated ones into the culti- 

 vated state led to their early occupation. Their formation 

 has, in manj' instances, led to the production of wet woods 

 and bouldery swamps in the intraglacial ground between 

 successive plains, as at Foxboro, Mass., on the Shore Line 

 Railroad, where the only available dry ground is a wash- 

 plain. 



In the suburbs of cities, the wash-plains afford vast 

 stores of gravel and sand used in the construction of 

 masonry and walks. The peculiar and regular structure 

 of these deposits makes it possible to give directions for 

 the search after sands and gravels. Coarse gravels will 

 be found in the ice-contact zone, normally the northern 

 aspect of the deposit, and in the top- set beds. The lobate 

 margins afford supplies of the finest sand which the de- 

 posit holds. From these observations, it follows that a 

 search for gravels should be begun at the northern side ; 

 for sand, at the southern side of a wash-plain. By strip- 

 ping off the top-set beds, a supply of moderately fine sand 



1 On the decomposition of rocks, see Rocks, Rock-weathering and Soils, by 

 George P. Merrill. New York, 1897, Part iii. 



