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 clay, 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 
