72 GEOLOGY OF THE NARRAGANSETT BASIN. 



deposits of the district to distances which vary with the endurance of the 

 particular kinds of rock. The crystalline limestones of Lincoln form trains 

 having a traceable length of from 4 to 5 miles; after a journey of that 

 length the rather soft bowlders seem to be quite worn out. The exceed- 

 ingly hard ilmenitic magnetite which occurs in Cumberland near Woon- 

 socket, Rhode Island, has yielded the most perfect bowlder train that has 

 yet been traced in this section of the country. Originating in a deposit 

 which has a width transverse to the path of the ice of only a few hundred 

 feet, this train extends in a gradually broadening path to the outer or 

 southern part of Narragansett Bay in a nearly north-and-south course; 

 thence it appears to have been deflected easterly, so that it overlapped 

 the western peninsula of Marthas Vineyard, known as Gray Head. In that 

 district four or five specimens of the unmistakable rock have been found, 

 which afford sufficient evidence that the train extended at least 60 miles 

 from the point of origin. 



In a description of the Iron Hill bowlder train, 1 I have given a detailed 

 account of its phenomena; and an estimate, based on such data as were 

 obtainable that served to show the amount of the rock in the deposit, was 

 that the amount of erosion which had taken place at Cumberland Hill 

 during the Glacial epoch was not less than 60 feet. In reviewing the facts, 

 it seems to be evident that this estimate is under rather than over the truth. 

 It is not unlikely that if all the waste from this elevation which was 

 removed by ice action could be restored, the summit would be near 200 

 feet above the present level. It is not to be supposed that the amount of 

 erosion in the Narragansett area was as great as that which occurred at 

 Iron Hill. At the time the ice began to act, that mass was probably at a 

 much higher level in relation to the surrounding country than it is at pres- 

 ent; it is likely that the processes of decay had penetrated deeply along the 

 numerous joints, so that when assailed by the ice it rapidly broke up. 

 However, making what seems to be all due allowance for this probably 

 greater erosion in this point, it must be confessed that, taken with the evi- 

 dence before adduced, it serves to show that a considerable thickness of 

 beds, perhaps near 100 feet of rock, must have been worn from this area 

 during the time the ice lay upon it. 



■The conditions of erosion beneath deep glaciers, based upon a study of the bowlder train from 

 Iron Hill, Cumberland, Rhode Island, by N. S. Shaler: Bull. Mus. Comp. Zool. Harvard Coll., Vol. 

 XVI, No. 11, January, 1893, pp. 185-225, 4 pis. and map. 



