146 LANCASTER D. BURLING 



and (8) the local downwarping along joint planes, to which this 

 paper is directed. 



The downwarping of certain layers near the top of the Lockport 

 dolomite has been the subject of frequent reference, and illus- 

 trations of it have been copied and recopied. The structure was 

 first described by Hall, who characterized it as concretionary. 1 

 Chamberlin and Salisbury 2 first called attention to the fact that 

 the sag is along joint planes, an explanation which is concurred 

 in by Hobbs, 3 but neither of these authors makes any comment 

 regarding the period of deformation. This was first treated by 

 Gilbert in 1905 in a paper of which we have only an abstract. After 

 describing the structures he says 4 that he is not satisfied with Hall's 

 characterization of them as concretionary, but that they were 

 probably contemporaneous with the deposition of the strata and 

 not subsequent to it. 



The phenomenon has been described for the following local- 

 ities: (1) Niagara limestone at Porter's quarry, Niagara Falls; 5 

 (2) Niagara limestone, Cook's quarry, near Lasalle, Niagara 

 County, New York; 6 (3) Lockport dolomite, Niagara Falls: (a) in 

 new railroad cutting; (b) in quarry 3 miles east of the city; and 

 (c) in water channels temporarily exposed at the Dufferin Islands 

 on the Canadian side; 7 (4) Lockport dolomite, Niagara Falls, old 

 quarry 1^ miles east. 8 



The purpose of this paper is to show that such structures are 

 essentially contemporaneous with the deposition of the strata, that 

 they should be expected to occur where they do, that they are 

 important, and that they have formed under essentially similar 



1 Geol. New York, Part 4 (1843), P- 94> Fig. 30. 



2 Geology (New York: Henry Holt & Co., 1904), I, 150, 151. 



3 Earth Features and Their Meaning (New York: Macmillan, 1912), p. 224, 

 legend to Fig. 239. 



« Science, N.S., XXI (1905), 224. 



5 Geol. New York, Part 4 (1843), p. 94, Fig. 30. 



6 Chamberlin and Salisbury, Geology (New York: Henry Holt & Co., 1904), I, 

 Fig. 137, P- 151- 



i Gilbert, Science, N.S., XXI (1905), 224. 



8 Kindle and Taylor, Geol. Atlas of the U.S., U.S. Geol. Survey, Niagara Folio 

 (No. 190), 1913, Illus., Ill, PI. XXIV. 



