156 bulletin: museum of comparative zoology. 



the usual kind found in the Roxbury formation : — granite, f elsite, 

 melaphyre, sandstone, quartzite, with some shale or slate fragments. 

 The slate fragments found so abundantly in the exposure at Squantum 

 farther south, and at Atlantic, are fewer here. A few may be seen 

 near the contact with the slate. Near the middle of the tillite on the 

 top of the hill there is an intercalated bed of water-laid gravel averag- 

 ing about twenty feet thick. This bed may be seen again at the point 

 of the Head on the north side. There is a north-south fault between 

 the two exposures. On the glacial hypothesis it is apparent that the 

 ice retreated and advanced again. On the shore to the north of a 

 quarry which is on the top of the hill, may be seen a bed of sandstone 

 about twenty feet thick and perhaps fifty feet from the bottom of the 

 tillite at its contact with the lower slate. iVs this sandstone comes 

 between two beds of tillite, it indicates another retreat and advance of 

 the ice. 



The fault mentioned above, cuts the tillite on a line near the front 

 of the barn at the end of the road, and may be seen at the place on the 

 shore where slate is encountered south of the dwelling house. Pro- 

 ceeding in a straight line from this point past the barn, the fault may 

 be located on the north shore. 



Dr. F. H. Lahee has observed plications in the slate south of the 

 tillite bed which deserve notice here. The plications occur in layers 

 of slate, and above and below such plicated layers the slate bedding 

 has not been disturbed. The upper parts of the folds have been cut 

 off, showing that the folding went on during the deposition. Dr. 

 Lahee suggests that floating ice became grounded and compressed 

 the layers, and later on when the same ice or other ice floated over 

 these layers, the tops were cut off. Prof. James Geikie (1895, p. 271- 

 274) has noted like plications in clay beds overlying the till at Porto- 

 bello, Scotland, and he suggested grounding ice-rafts, as Lahee did, for 

 the plicating agency. I have noted the same kind of folding in slate 

 at Crow Point, Hingham, and at the Chestnut Hill fault locality, on 

 Beacon Street west of Hammond Street, Chestnut Hill, but have not 

 seen evidence of the cutting off of the folds at these localities. At 

 the Atlantic exposure also folds in slate may be seen, with arches 

 cut off as at Squantum Head. It is not impossible that the tops of 

 these folds were removed by a swifter flow of water, as evinced at 

 Atlantic by ripple-mark of fine sandstone above the folds. The 

 same kind of folds, but not cut off, may be found in Pleistocene clays 

 in many places in this country. Near Hanover, N. H., I have found 

 many folds of this description. In view of the fact that the layers 



