ABSTRACTS AND DISCUSSIONS OF PAPERS 111 



the main tillite. One of these beds is about 50 feet from the main tillite 

 formation and the other about 150 feet. Prof. John E. Wolff has made a 

 microscopical examination of the tillite and slate and finds that they have the 

 same nrineralogical composition. As one goes upward in the banded slate it 

 is seen that the layers of sandstone become thinner and thinner, then disappear 

 entirely, and finally the alternating layers are shown only in slate of dark and 

 light bands, the dark bands containing more heavy basic material than the 

 light. The cause of this disappearance of the sandstone layers can be ex- 

 plained by the gradual withdrawal of the glacier and the consequent slacken- 

 ing of the currents, which would be strong enough to carry sand only in the 

 neighborhood of the ice. The lower pebbly members of the transition beds 

 show irregularities of deposition, especially in lenticular forms, due to the in- 

 constant conditions of streams coming from a glacier. A regularity of alterna- 

 tion in deposition, however, becomes evident after the first 50 feet or so of the 

 transition beds have been passed, where the layers indicate deeper and quieter 

 water, and thus more uniformity in deposition. The thin individual layers of 

 sandstone and slate now show through hundreds of feet such regularity in 

 thickness and interval that a regularly recurring cause must be sought. 



Professors P>. K. Emerson and Gerard De Geer have described this same 

 regularity of layers of sand and clay in the glacial clays of the Connecticut 

 Valley and Sweden respectively and have ascribed the banding to yearly 

 deposition. They believe that the layers of coarser material indicate summer, 

 when the streams from the melting ice were rapid, and the layers of fine 

 material indicate winter, when the streams had less velocity. Neither Emer- 

 son nor De Geer could suggest any other period to which such regular alterna- 

 tions could be related. Other American geologists have published this same 

 idea which De Geer conceived in 187S. Among them may be mentioned Dr. 

 Charles P. Berkey, Prof. A. P. Coleman, and Mr. Frank B. Taylor. This ex- 

 planation may not be generally accepted, but no other cause, so far suggested, 

 accounts for all the facts of the case. When the seasonal units have been 

 accurately determined, they will furnish a basis for estimating the length of 

 time required for the deposition of the formation. 



The facts observed in the slate at Squantum resemble so closely those 

 described by De Geer and Emerson that it would seem as if there must be a 

 very strong probability of these being similar cases, in spite of the millions of 

 years which separate the two Glacial periods. 



Bead in full from manuscript. 



Discussion 



Prof. W. W. Atwood : This paper appeals to me as particularly interesting. 

 I have seen similarly banded clays associated with the Wisconsin drift and 

 during the past season I found just such banded clays associated with the 

 Eocene till of southwestern Colorado. In examining in the field the slates 

 here described by Mr. Sayles I was convinced that the regularity of the repe- 

 tition of the bands of sands and clays meant a regularly returning cause, and 

 seasonal changes in stream action seem adequate. This paper suggests that 

 the idea of seasonal changes in deposition so well put forth by De Geer and 

 others may help us in working out a calendar of ancient geologic time and 

 IX — Bull. Geol. Soc. Am., Vol. 27, 1915 



