MODIFICATION OF GREAT LAKES BY EAETH MOVEMENT. 357 



when there is little wind. The effects of light winds can be approxi- 

 mately eliminated by taking the average of many observations, and so 

 can the effects of seiches and tides. The effects of differences of atmos- 

 pheric pressure can be computed from barometric measurements of air 

 pressure, and the proper corrections applied. It is also possible, by 

 the discussion of long series of observations at each station, to deter- 

 mine the local tidal effects and afterwards apply corrections; and the 

 laud and sea breeze effect may be treated in the same way. 



In the investigation I was able to make, consideration was given to 

 these various sources of error, but it was not practicable to take all 

 desirable measures for avoidance or correction, because the reading of 

 gages was only partly under my control. Gage stations have been 

 established on the Great Lakes at various times and at various places, 

 and the records of readings have been preserved. In some cases the 

 zeros of gages were connected by leveling with bench marks of a 

 permanent character, and in a few instances the gages themselves are 

 stable and enduring structures. The most important body of informa- 

 tion of this character is contained in the archives of the United States 

 Lake Survey, which were placed at my service by the Chief of Engi- 



B 



c 



7777777777770/ 



777777777777777T77m777777777W7//////////////7///// 

 Fig. 6. 



DIAGRAM ILLUSTRATING THE METHOD OP USING A LAKE SURFACE FOR THE DISCOVERY AND MEASURE- 

 MENT OF EARTH MOVEMENTS. 



neers, United States Army. By searching the records I was able to 

 select certain pairs of stations at which the relative heights of 

 X^ermanent points on the shore (equivalent to A and B of the diagram) 

 had been practically determined twenty or more years ago. At some 

 of these stations gages are still read; at others I established gages 

 and ran the leveling lines necessary to connect them with the old 

 benches. At all of them observations were maintained from July to 

 October, 1890, and these observations, in combination with the 

 levelings, afforded measurements that could be compared with those 

 made earlier, so as to discover changes due to earth movement. 



It will not be necessary to give here the details of observation and 

 computation, as they are fully set forth in a paper soon to be printed 

 by the Geological Survey, 1 but the general scope of the work may be 

 briefly outlined. As the tilting shown by the geologic data was toward 

 the south-southwest, stations were, so far as possible, selected to test 

 the question of motion in that direction. The most easterly pair were 

 Sacketts Harbor and Charlotte, New York, connected by the water 

 surface of Lake Ontario. (See map, fig. 7.) From observations by the 

 United States Lake Survey in 1874, it appeared that a bench mark on 



U. S. Geol. Sury., 18th Ann. Kept., Part II, pp. 595-647, 1898. 



