SAYLES: THE SQUANTUM TILLITE. 167 



feet along the shore. Of this exposure only the uppermost part will 

 be considered. 



Commencing on the little high-tide island opposite the end of the 

 road, thin intercalated beds of sandstone and slate are found. One 

 of these beds has a plication in an east-west direction which may have 

 been made by ice-push. It does not seem probable that this plication 

 was caused by diastrophic movement, not only because the movement 

 was at right angles to the main direction of folding, but also because 

 there are no signs of plication above or below this bed. It is of course 

 possible that there was diastrophic movement transverse to the main 

 direction of folding, but if this had been the case here it would seem 

 that there should be some evidence of it above and below the plication. 



Above this first intercalated bed, near the top of the tillite, there are 

 two more similar beds, and between each, undoubted tillite. In places 

 there are very fine layers of slaty material not more than one sixteenth 

 of an inch thick. Pebbles are pressed into these thus cutting them off 

 and deforming them. These tiny clay-threads suggest melting of the 

 ice and trickling of water laden with clav, between the ice and the 

 till. 



A large block of pink granite, in the tillite on this island, six feet 

 long and one foot wide, is important in showing transportation with- 

 out wear. (Plate 9). The block is angular. It is not easy to see 

 how this block could have been transported in its present fresh con- 

 dition by any other agency than an iceberg or a glacier. 



Returning to the main land and proceeding in a southerly direction 

 along the shore, the transition-beds from the tillite to the main slate- 

 body can be studied with ease. The beds intercalated in the tillite 

 grow in thickness towards the top, suggesting longer retreats of the 

 ice each time. The proportion of pebbles to matrix increases, and 

 slate fragments of all shapes and sizes make their appearance. The 

 tillite now suggests very thin ice acting for short periods, for the peb- 

 bles are very abundant. Retreats and advances were of shorter 

 duration. The reappearance of the slate fragments at the top of the 

 tillite is to be explained, I believe, in these advances and retreats of 

 the ice. The ice retreated, and deposits of gravel, sand, and clay 

 were made on the ground left vacant by the retreat. Again the ice 

 advanced, ploughing up the beds formed at its front and making a new 

 till composed of parts of gravel, sand, and clay-beds. 



Disrupted sandstone and slate beds come above this slate lump 

 horizon, and then appears the main body of the slate, the highest 

 member of the series in the Boston Basin. The ice had then retreated 



