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, ha\^e 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 m.ight 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. 
