470 JAMES WALTER GOLDTHWAIT 



easily accounted for. It is a beach ridge on a height of land, where 

 no opportunity exists for the record of a higher stage. The other 

 two discordant points, one at Petoskey and one at Leland, are five 

 feet below and 4 feet above the center of the band, respectively, 

 instead of being within 3 feet of it. This discordance of one or two 

 feet is not a serious one; for it might be due to an error in leveling 

 which augments an original variation in height, instead of being 

 included in that variation. 



The rate of inclination of this Algonquin plane measured from 

 Hessel (at the ancient " Munuscong Islands") southward to Mackinac 

 Island (15 miles) is 3.73 feet per mile. From Mackinac Island to 

 Beaver Island (24 miles) it is 3.30 feet per mile. A rather rapid 

 change of inclination near the isobase of Beaver Island introduces 

 a tilt rate of about 2.00 feet per mile. Over the southern part of 

 Grand Traverse Bay the rate again decreases, perhaps more gradually 

 than in the former case, so that near Traverse City it is about i.oo 

 foot per mile. The further change from an incline to a horizontal 

 position, which is accomplished near Onekama, seems to be a rather 

 rapid one; for Onekama is only 25 miles south of Traverse City, 

 where, as we have just remarked, the tilt rate is one foot per mile. 

 The abruptness of the changes near Beaver Island and Onekam 

 could be emphasized by representing the plane on the profile by a 

 bent line rather than a curved one. The curve has been used here 

 merely for simplicity, without meaning to imply that the deformation 

 is necessarily a warping rather than an uplift by the tipping or jostling 

 of large fault blocks. Either sort of uplift seems admissable, when 

 due weight is given to the opportunity for variation in the height 

 of the beaches or benches. 



Below the highest shore line are a number of others. Some of them 

 are equal to the Algonquin in strength; others are comparatively 

 faint. They can best be recognized in the northern part of the 

 region, where the vertical space between them is greater. On Mack- 

 inac Island, for instance, strong benches and beaches record ten 

 lower stages of considerable importance. The Algonquin beach is the 

 highest of a closely spaced group of ridges which are well displayed 

 on the short target range back of Fort Mackinac. This "Algonquin 

 group" of beaches occupies an interval at that place of about 50 feet. 



