112 BULLETIN OF THE ESSEX INSTITUTE. 



Professor Shaler, in his report on tlie Geology of Nan- 

 tucket, has presented a study of decay in the glacial de- 

 posits of that island the leading features of which changes 

 I cite in his own words :^ 



" Perhaps the most noteworthy feature in these deposits 

 of drift is the very extensive decay to which the pebbles 

 and sand have been subjected. Some of the consequences 

 of this decay will be noted below. In their form and 

 structure, the drift deposits in no distinct way differ from 

 the similar accumulations found in the region a hundred 

 miles farther north, but in their state of preservation they 

 present important differences. The decay which has at- 

 tacked the pebbles is exhibited in the following ways, viz. : 

 (1) By the interstitial decay of the stone, which mani- 

 fests itself in the crumbling of many of the varieties of 

 crystalline and fragmental rocks; (2) by the dissolved 

 look of the surface of the rocks which resist the intersti- 

 tial decay; and (3) by the development of the incipient 

 joint planes in the pebbles, so that, though they may be 

 but little decayed, they often split into fragments on be- 

 ing removed from their bed." 



An examination of the pel)bles in some of the wash- 

 plains near Boston shows the presence of similar effects due 

 to chemical action. The most conspicuous example which 

 has fallen under my notice is the case of the overridden 

 deposit or"karae"on the west side oi Fresh Pond, in 

 which thousands of pebbles break down into angular pieces 

 or have been so far leached out as to crumble into a rusty 

 red powder when released from the bank. 



In the Woodland wash-plain, the following changes in 

 the section lying above the water-plane in the gravels have 

 been observed. In the first place, pebbles lying near the 

 surface of the deposit in the top-set beds and having 



1 Bull. 53, U. S. Geol. Survey, pp. 21-22. 



