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NEW YORK STATE MUSEUM 



on the Alexandria sheet, what appears to be a quite similar broad 

 belt of limestone borders the west side of Butterfield lake. It is 

 however so much concealed by overlying Potsdam sandstone that 

 some uncertainty attaches to its extent and purity. But it has a 

 breadth of outcrop quite comparable to that of the Indian river 

 belt, and seems to consist chiefly of pure limestone. Its dips are 

 prevalently to the southeast, and somewhat steeper than in the 

 previous case, averaging 6o°. This means a thickness substantially 

 the same as in the other case, and strongly suggests that the two are 

 parallel outcrops of the same great limestone belt, and that, since 

 they dip toward one another, the structure is synclinal. If this 

 be the true interpretation then the schists, amphibolites and quart- 

 zites which lie between the two limestone belts, rest on the limestone 

 and hence are younger, with the rather massive quartzites about 

 Sixberry and Millsite lakes as the youngest of all ; while the schists 

 to the northwest on the Alexandria quadrangle, and to the south- 

 east on the Theresa quadrangle, underlie the limestone and are 

 older. Figure 7 will illustrate the suggested structure. 



; Ctf 7 C ~> S 



Fig. 7 Section to illustrate the structure suggested by the Grenville rocks, on a scale of 

 4 miles to the inch; s=schists, c=crystalline limestone, q=quartzite 



There are, however, two alternative views in regard to this struc- 

 ture which may be held. It is possible that these two thick lime- 

 stone masses may be separate beds, the one overlying the other and 

 separated from it by the thickness of schist and quartzite which 

 lies between. This involves the assumption that the series, though 

 greatly tipped, is not folded and hence that no bed is cut by the 

 present surface along more than one line. Since, however, small 

 folds are certainly present in considerable number, the changing dips 

 indicate the presence of greater ones, and as we have here two 

 great lines of limestone outcrop, the rock showing much the same 



