A TEST OF ISOSTASY 305 



writer to the hypothesis of plastic deformation which he has lately found 

 to have been already formnlated by Jamieson. Certain facts observed in 

 Xew England seem to be explicable only on the assumption that there has 

 been offshore subsidence in Eecent time. If that subsidence was cor- 

 relative with the post- Glacial uplift of northern New England, the as- 

 sumption enters the domain of Jamieson's hypothesis. If his speculation 

 is well founded, similar subsidence in the belt surrounding the former 

 ice-cap west of N'ew England should be expected. Can the value of his 

 speculation be tested by those expert in the geology of the long belt in- 

 volved? The question can be satisfactorily answered only by the pooling 

 of conclusions reached by many specialists. The present writer must 

 be content with listing examples of specific or local tests, which, however, 

 must be applied by those more competent. 



Evidence of isostatio Movements off the New England Coast 



isobases 



During recent years the writer has studied, at intervals, the emerged 

 coastal strip from Boston to Boothbay Harbor, Maine. In agreement 

 with Tarr, Woodworth, and Laforge, he believes that the highest post- 

 Glacial shoreline is located on Cape Ann at about 80 feet above the present 

 sealevel. It is a few feet lower at Magnolia, Massachusetts, but rises 

 rapidly to the northward, reaching a height of nearly 300 feet near Booth- 

 bay Harbor, 115 statute miles from Magnolia. Katz has determined its 

 height as 155 feet at Stratham, New Hampshire, and as 300 feet at 

 Pownal, Maine, 26 miles west of Boothbay.^ According to Katz, the 

 shore structures "indicate that the postglacially uplifted surface has 

 been tilted 5 to 6 feet per mile in a direction north 40° east of south, 

 and that the lines of equal elevation approximately parallel the shore 

 of the Gulf of Maine.'^ These results accord with De Geer's isobases for 

 the region, but are contrasted with the recent conclusions of Fairchild, 

 whose isobases portray a post-Glacial uplift of this coastal belt which is 

 200 to 300 feet too great.* 



WEAK WAVE EROSION IN THE EMERGED ZONE 



Though the emerged belt of New England shows the well known fea- 

 tures — wave-washed rock slopes, raised beaches, sand and clay plains — the 



3 R. S. Tarr and J. B, Woodworth : Bull. Museum Comp. Zoology. Cambridge, Mass. 

 vol. 42, 1903, p. 181. 



F, J. Katz : Proc. Washington Acad. Sci., vol. 8, 1918, p. 410. 

 * G. De Geer : Proc. Boston Soc. Nat. Hist, vol. 25, 1892, p. 454. 

 H. L. Fairchild : Bull. Geol. Soc. Am., vol. 29, 1918, p. 202 ; vol. 30, 1919, p. 614. 



