170 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
therefore, in regard to these two beds in the tillite is, that they prove 
milder conditions and temporary retreats of the ice-sheet, and that the 
cause of glacial periods fluctuated in the distant geological past much 
as it did during the Pleistocene period. It would be a difficult matter 
even with the aid of fossil plants, to prove an extended interglacial 
epoch in such a limited deposit as that found in the Boston Basin, 
unless other evidences of interglacial conditions were present. 
ACKNOWLEDGEMENTS. 
It was through Dr. Laurence La Forge that I first saw the rocks 
discussed in this paper: He pointed out to me localities 1, 2, 5, 7, 12, 
14, 15, 17. During trips with him I obtained a clear insight into 
the structure and stratigraphy of the rocks in the Boston Basin as 
determined by him. Subsequent research independently corroborates 
the ideas he held in 1910, with the exception of his interpretation of 
the structure of Squantum. The light which my recent work on this 
locality has thrown, obliges me to differ from him in regard to the 
structure of this section of the field, and the reasons for my difference 
of opinion have been given in the description of Locality 13. Prof. J. 
B. Woodworth has given me invaluable criticism and advice. Ac- 
knowledgements are also due to Profs. W. M. Davis, A. C. Lane, 
J. E. Wolff, Ellsworth Huntington, and George R. Mansfield, to Drs. 
F. H. Lahee, and Arthur Keith, to Mr. Burton M. Varney, and Mr. 
George M. Flint. 
