SOME GLACIAL WASH-PLAINS. 115 



tions of chemical waste such as give rise to sands ami 

 gravels cemented by iron oxides in other Helds. In fact, 

 there are few or no instances in which t\w consolidation 

 of considerable masses of the glacial gravels have been 

 observed in this area. Such consolidation as I have ob- 

 served has most conspicuously taken place in a series of 

 gravels and sands antedating the last glacial advance as on 

 Martha's Vineyard and Block Island. 



The result of the loss of materials in the upper parts 

 of our glacial sand-plains by chemical solution must in 

 the end become ap[)arent in the lowering of their mass. 

 If the action is uniform, the skeleton pebbles will crush 

 and settle down into the open spaces below. Owing to 

 the openwork structure of the gravels, the falling of the 

 decayed pebble matter into the spaces remaininir between 

 the sound quartzose pebbles might lower the surface of 

 the ground several feet. Since the pel)bles and the conse- 

 quent openwork structure are mainly developed at the head 

 or in the ice-coutact zone of the plains, this part will under- 

 go the orreater amount of settlins: from solution and crush- 

 ing of the skeleton pebbles. For this reason important 

 topographic bench marks should not be located upon the 

 table-like deposits of this class nor should permanent and 

 weighty stone structures be built upon these terraces. 

 The falling in of the surface of these deposits, if it should 

 occur, can hardly be discriminated topographically from 

 the eflects of the pronounced caving in which took place 

 shortly after their deposition from the melting out of 

 masses of ice. 



The rate of solution of carbonate of lime under the 

 conditions in which it exists in these wash-plains has not 

 l>een determined. It is quite certain that, when the peb- 

 bles were brought to their position in the deposits, the 

 veins of carbonate of lime were intact. We can ascer- 



E8SEX INST. BULLETIN, VOL. XXIX 8* 



