574 RECORD OF MEETINGS OF THE 



serpentine. In thin sections from the upper part of the pla- 

 teau, and in close proximity to the overlying amphibolite, there 

 appear shredded masses presenting the original structure of 

 hornblende as seen in the amphibolite, but mineral ogically 

 altered to a fibrous mass with the optical characteristics of 

 anthophyllite. It is not improbable, moreover, that a portion 

 of the hornblende has altered to tremolite. These fibrous 

 constituents form the so-called "slip-fibre." 



The serpentine belt has also been subjected to peculiar fault- 

 ing and crushing. The cracks thus produced, even on a micro- 

 scopic scale, have been filled with these fibrous constituents 

 and then the whole mass submitted to further slipping. This 

 has caused the slickensiding phenomena on the fracture planes 

 and a consequent stretching of the fibrous content; hence the 

 term "slip- fibre". " Cross- fibre " or true thrysotile is to be 

 found in this area. It is best developed along lines of maxi- 

 mum fracture and minimum lateral thrust. There appear to 

 be two bands of maximum fracture, one stretching along the 

 upper portion of the plateau and not far from the garnet 

 zone, the second along the foot of the plateau and best shown 

 on the property of Judge Tucker. 



Dr. Berkey said that laminated clays of Glacial and Post- 

 glacial age are abundant in many districts of the Northern 

 States and Canada. They are especially abundant about the 

 head of Lake Superior, where the origin of the deposits is 

 intimately related to the closing fluctuations and final with- 

 drawal of the Wisconsin ice-sheet. 



One of these deposits at Grantsburg,Wis., exhibits a remarkable 

 uniformity of structure and is so clearly bounded by other accu- 

 mulations of known significance that its history is readily inter- 

 preted. From a detailed analysis of its laminated structure it is 

 argued that this deposit was about 1700 years in accumulating. 



A like interpretation of similar isolated deposits following 

 the retreating ice-sheet would give data for time estimates 

 from an entirely new standpoint. In some areas laminated 

 clays occupy interglacial positions, and it may be possible to 

 apply the same method to them. 



The last paper of the evening was by Professor Grabau, on 



