1871.] 91 [Pickering. 



Dr. Charles Pickering made a verbal communication on 

 the "drift" and especially the examples of it, which he had 

 carefully studied in Salem. These he explained by diagrams 

 on the blackboard. 



He said Prof. Agassiz's idea of a great mass of overlying ice 

 seemed at first to account for the movement of the boulders near 

 Salem, but there are circumstances that hardly correspond. Once 

 enveloped in ice, fragments of rock, whether large or small, would be 

 all treated alike, whereas the largest of the Salem boulders are pretty 

 regularly left near their source; water, as suggested by Dr. Jackson, 

 would make this discrimination, but there is more resemblance to 

 the probable effect of a slight check of the earth's rotation, sending 

 loose material eastward. 



The source of supply is clearly defined, a bed of syenite extending 

 along the west side of Tapley's brook from Dan vers to Saugus; the 

 derived boulders are scattered over a bed of greenstone-trap, from 

 Tapley's brook eastward to the sea ; and the exact amount of force 

 expended in transporting each boulder to its present position can be 

 mathematically demonstrated. 



Neither beyond nor around Salem had he ever met with a frag- 

 ment of conglomerate, even in the finer '' drift." Eight miles farther 

 south, on the edge of Little Nahant, he had found a large boulder of 

 porphyry-conglomerate, that came, in all probability, from the hills 

 between East Boston and the sea, and therefore nearly due east. 



These boulders having been transported by some cause that has 

 ceased to operate, a question arises as to the date. Was it anterior 

 to the advent of the existing species of animals and plants on the 

 surface of the earth? 



Mr. Hyatt stated that he had recently explored the region referred 

 to by Dr. Pickering, and could confirm the statements of that gen- 

 tleman, so far as they related to the limited distribution of boulders. 

 The geology of that vicinity was of great interest, and had been 

 the object of several of his excursions during the past summer. He 

 then described the valley which leads southward from Peabody to 

 Swampscott, as having been at one time filled by a vast body of 

 eruptive syenite. Remnants of this are still to be found capping the 

 summits or forming the south-western face of the ridge on the 

 eastern side, while on the western side they project in numerous, 





