18 TWELFTH Al>rNUAL REPORT. 



tion of general Q. A. Gillmore whose previous experiments and 

 publications have made him one of the best authorities in the United 

 States, if not in the world; and that in consequence of this phen- 

 omenon he had special trials made, and new instruments prepared, 

 yet with the final results stated above. It must be admitted that 

 previous tests, made at the same place (Fort AYadsworth, Staten I.), 

 on the stones at Duluth, Saint Cloud and Lemont, giving less com- 

 pressive strength to those stones than now reported, throws a 

 shadow of doubt on the correctness of the methods employed. It 

 may be possible to explain those three cases in some way satisfac- 

 torih', by referring them to imperfections in the cubes. It is cer- 

 tainly not possible to allow them to establish a rule, in the face of 

 twenty other samples which contradict them. 



(3). Are the granites of Minnesota stronger than those of New 

 England? We must either allow this, or, on account of the care- 

 fulness of the late tests of Minnesota granites, we must impugn ail 

 the results and reasoning published heretofore by general Gillmore 

 on the granites of New England. Allowing this, we may speculate 

 as to its possible cause. 



It had occurred to me prior to this investigation, from other con- 

 siderations, that perhaps the last glacial movements in Minnesota 

 were of a later date than those described in New England. The 

 evident freshness of the drift in Minnesota, in its 2)ose, and espe- 

 cially of the till, compared with that of southern New England, 

 and southeastern New York, seems to indicate the same differences 

 as to time, of deposit, as can be inferred between the northern and 

 the southern portions of the state of Ohio^ , or the same portions of 

 the state of Minnesota. Of course, the continuous tracing of the 

 same lines of morainic accumulations from east to west will finally 

 determine the eastern analogues of our Leaf Hills and Kettle mor- 

 aines, and will give a definitive answer to this hypothesis. In the 

 mean time, and before that is accomplished, we may perhaps account 

 for the greater strength of Minnesota crystalline rocks by suppos- 

 ing them less changed superficially by the process of decay, the 

 lateness of the glaciation to which they have been subjected having 

 left them comparatively fresh through the recent removal of a con- 

 siderable thickness. 



1 Geological survey of Ohio. Report on Delaware county. 



