112 PROCEEDINGS OP THE WASHINGTON MEETING 



lead to a fuller appreciation of the significance of varying climatic conditions 

 on sedimentation. 



Professor Hobbs : I have been greatly interested in Mr. Sayles' able dis- 

 cussion of this subject and the conclusions which he has reached concerning 

 the annual deposits indicated by the tillite. It was my good fortune to ex- 

 amine with Baron De Geer a number of sections in the thin-banded clay in 

 the delta deposits along the former ice-front of Scandinavia. Checked as 

 these individual bands were by moraines of annual deposition, and worked out 

 as they have been with such infinite care, one could hardly fail to accept the 

 conclusion to which they pointed. The conclusions of Mr. Sayles point the 

 way for many studies in the future which it is certain will be crowned with 

 results. I was interested to note that in some of his sections there was indi- 

 cation of what represents, perhaps, a climatic cycle, something entirely to be 

 expected, though it did not seem to be indicated by the banded clays near 

 Stockholm. 



Prof. Joseph Barbell : Doctor Sayles has given a very convincing presenta- 

 tion, showing that the banding in the argillites associated with the Squantum 

 tillite corresponds to the similar banding in Pleistocene glacial clays and is 

 to be interpreted as showing seasonal variations in the Permocarboniferous 

 glacial climates. This is both interesting and important, since the biologic 

 evidence of most periods previous to the later Mesozoic shows warmth without 

 marked zonal climates extending into high latitudes. 



In the combinations of winter and summer conditions which go to make up 

 the geologic climates, there are four combinations possible, so far as warmth 

 and cold are concerned : First, winter may be submerged and summer is domi- 

 nant throughout the year, with or without a season of drought. This is the 

 present climate of the torrid zone and in a number of past times appears to 

 have been the dominant type even into high latitudes. It means the existence 

 of some condition which carried solar heat received in equatorial latitudes 

 into high latitudes with minimum loss by radiation. Second', winter and sum- 

 mer may alternate, but with mild winters, the present climates of the warm 

 temperate latitudes. This was perhaps the commonest type of climate in high 

 latitudes during the Jurassic and Cretaceous periods. Third, winter and sum- 

 mer may alternate, but with rigorous winters, leading, with sufficient snow- 

 fall, to glacial conditions. This is the climate typical of high temperate lati- 

 tudes today, but in the Pleistocene it extended into middle latitudes. It is 

 favored by direct and local absorption of solar heat and its re-radiation, with 

 minor control by a planetary distribution of equatorial warmth. It seems, as 

 Doctor Sayles has here shown, to have been the type also of the Permocar- 

 boniferous glaciation in eastern Massachusetts. Fourth, the climate may be 

 continuous winter, as seen loithin the present ice-sheets in polar latitudes — 

 summer completely submerged, winter continually dominant. 



Now, regarding the obliquity of the ecliptic and position of the pole as un- 

 changed through geologic time, the middle latitudes should show the greatest 

 contrast between the total amounts of winter and summer insolation. Doctor 

 Sayles' paper suggests, in accordance with this expectation, that glaciation in 

 all times in the temperate zone has been connected, as at present, with a 

 marked seasonal climate, exhibiting the third of the several combinations 

 listed — a climate distinguished from normal geological climates by the emer- 

 gence of winter. 



