Notices of Memoirs — Prof. Shaler. 27 



HsroTiODES OIF" 3y^:E:M:oII^s. 



I. — On the Parallel Eidges of Glacial Drift in Eastern 

 Massachusetts. By Professor N. S. Shaler. 



(Proc. Boston Society of Nat. Hist., vol. xiii., February, 1870.) 



IN the immediate neighbourhood of Boston the unstratified Drift 

 does not lie in anything like a continuous sheet, but is dis- 

 tributed in long and rather narrow ridges, which, with varying 

 height, on account of long-continued denudation, may be traced for 

 miles across the country. These ridges are particularly conspicuous 

 in the islands of the harbour of Boston, where, although much worn 

 by the action of tidal currents, the parallelism is quite apparent. 

 They exhibit two sets of trends, the one being north-west and south- 

 east, the other north-east and south-west ; and a comparison of the 

 sections, given at various points in the islands of the harbour, at 

 Chelsea, Somerville, Cambridge, Brighton, South Boston and else- 

 where, has shown that throughout this region the Drift is remarkably 

 similar at the same height above the sea. 



The Drift contains pebbles of various sizes, five-foot boulders, and 

 fragments of every gradation down to coarse sand. The whole is 

 imbedded in a fine mud, which so binds the materials together that, 

 in the lower parts of the mass, where it has been subjected to con- 

 siderable pressures, it has become a hard conglomerate. Nowhere 

 is there any attempt at stratification. 



In regard to the origin of these Drifts, Professor Shaler agrees 

 with Agassiz in considering them as the materials which rested in and 

 upon the glacial sheet at the close of its history, and which were 

 dropped in the places they now occupy by the melting of the ice 

 upon which they rested. As this Drift deposit must have originally 

 been at least one hundred and fifty feet thick, it is difficult to con- 

 ceive how such a mass of detritus as that in question could have ever 

 been contained in a glacial stream, and the supposition is necessary 

 that the mass of the Drift must have been rent from the floor of the 

 glacier as it moved along. 



But there is evidence, according to Professor Shaler, that the 

 glacial sheet was at many points over half a mile in depth, and, 

 therefore, this action may readily be conceded to it. 



Moreover, in a section exposed at Cambridge, large masses of the 

 clay slate, grooved and scratched by long working on the solid rock, 

 were found in the Drift, at a height of several feet above the bed 

 from which they had been torn. 



It follows, as the author observes (if these conclusions be ac- 

 cepted), that this deposit of detrital matter must have covered, with 

 something like uniformity, the whole of this part of the coast ; and 

 indeed the relation of the separate masses of Drift is irreconcileable 

 with any other hypothesis — for they could not be the terminal or 

 lateral moraines of a glacier. 



The cause of the peculiar parallelism of the two series of ridges 

 is difficult to arrive at ; but it is known that the district around 



