14 BULLETIN OF THE LABORATORIES 



other minor points might be mentioned which bear on the subject, but 

 as this is not intended as an exhaustive treatise, they will be omitted 

 for subsequent consideration. The facts presented indicate that the 

 rocks of one quarry, those represented by Plates A and B, have been 

 subjected to a tremendous pressure, and the others have not, or at 

 least not to such an extent. That the pressure in the first case was 

 from a superposed burden and not a lateral force. And it is evident 

 that the burden has been subsequently removed. We believe that 

 the first named quarry was covered by the ice sheet, and that the sec- 

 ond quarry considered was not so covered, and would suggest it as a 

 possible means of determining the extent of the ice front in re- 

 gions where the country rock is easily fractured and presented in 

 erosion hills. 



As far as our observations on other exposures have extended we 

 think they sustain us in our views, those within the glacial region show- 

 ing evident marks of fracture by superposed burden and those beyond 

 the recognized limits showing no such evidence. From the curved 

 forms of the fracture spoken of in Plate A, and shown in many other 

 instances in a more or less decided manner, it has been suggested that 

 data might be derived by which the thickness of the ice, or at least 

 the amount of the breaking force might be determined. Apparatus 

 is now under construction for studying the resistance of blocks of the 

 rock of different dimensions to a crushing force, and the forms of the 

 fractures produced. There are many difficulties to be overcome and 

 many modifying influences, but it forms at least a very inviting field 

 for investigation. The illustrations are all from photographs taken 

 and prepared by the author. 



