462 JAMES WALTER GOLDTHWAIT 



beach, however, is the only point which one can take as a criterion for 

 measuring a water plane; so in the study of these raised beaches it is 

 the crest that has been measured each time. Care was taken to make 

 sure of the presence of gravel on the surface of these beaches, in order 

 to eliminate the effects of wind-blown sand, which often raises a 

 beach, locally. If we allow a range of five feet for original variation 

 in height of beach crests, we have probably satisfied all discordances 

 except those which can be recognized as due to peculiar local con- 

 ditions. 



Other varieties of the beach need scarcely be mentioned, such as 

 the bar or barrier, built between headlands, in comparatively deep 

 water. Its height is liable to be more extreme on that account. The 

 object in the foregoing remarks is to show that the points selected for 

 measurement (the base of a bluff, or the crest of a beach or barrier) 

 were chosen for convenience, not to say from necessity, and that an 

 original constructional variation in height of about five feet is fully 

 recognized. 



METHOD OF INVESTIGATION 



In the measurements the spirit- or Y -level was used almost exclu- 

 sively, and to decided advantage. While much has been accom- 

 plished with the hand level and aneroid, in skilful hands, neither of 

 these instruments has the accuracy or reliability of the Y -level. The 

 influence of weather conditions, especially lake breezes, on the 

 aneroid, and the personal equation in the hand level are liable to cause 

 mistakes in such work as this. Where there is a whole series of shore 

 lines to be measured in one locality and these beaches and benches 

 follow one another in short vertical intervals, all possible accuracy in 

 measurement is needed to correlate them, individually with mem- 

 bers of a similar series at a neighboring locality. The Y -level, 

 then, is almost indispensable for work in the central and southern 

 portions of the Great Lake region, and desirable in all parts of it. 

 Upham and Tyrrell used the Y -level ^ in measurements of altitude 

 of the raised beaches of Lake Agassiz before 1890, Spencer^ used it in 



1 Warren Upham, "The Glacial Lake Agassiz," U. S. Geol. Surv. (Monog., XXV), 

 p. 9, 1896. 



2 J. W. Spencer, "Deformation of the Algonquin Beach and Birth of Lake Huron," 

 Am. Journ. Sci. Vol. cxli, pp. 12-21, 1891, and other papers. 



