236 



NEW YORK STATE MUSEUM 



Chalmers describes small rock fractures of postglacial date 

 in southern Quebec like those above mentioned near Albany, and 

 the same kind of rock movement breaking glaciated surfaces has 

 been reported in New Brunswick by G. F. Matthew. 1 It is evi- 

 dent that the changes of level which have taken place in this 

 region have been accompanied by the local apparently widely 

 distributed faulting of ancient rocks. It is hoped that special 

 investigation of these dislocations in the upper Hudson valley 

 will give data for applying a correction to the local data on which 

 the recognition of the present attitude of the ancient water levels 

 depend. 



BEARING OF CHANGE OF LEVEL ON THE DURATION OF THE POSTGLACIAL 



INTERVAL 



The elevation of shell-bearing beds at Montreal on Mt Koyal 

 to a hight of 550 feet together with the existence of the ancient 

 marine limit marked by beaches at an altitude of 450 feet near 

 the international boundary on the north slope of Covey hill, 

 affords a basis for the calculation of the time which has elapsed 

 since the marine shore lines were level water lines, provided the 

 rate of tilting and local elevation can be satisfactorily determined. 

 The fact shown in plate 28 and discussed in the preceding pages 

 that an upper water level, apparently of lacustrine nature declines 

 at a rather uniform rate from north to south in the Champlain 

 region makes it evident that the assumption of any given 

 rate of vertical movement e. g. 3 feet a century would be erroneous 

 for all except one point in the elevated district. Far to the north 

 of the international boundary in the Hudson Bay district, Bell 

 has given his reason for thinking that elevation is now taking 

 place at a rate between 4 and 5 feet a century. On the south along 

 the coast at the mouth of the Hudson river, Cook and others have 

 estimated that the coast is now being depressed in relation to 

 sea level at the rate of 2 feet a century; but recent engineering 

 measurements of tidal range show according to Mr George W. 

 Tuttle 2 that at New York city since 1875 the subsidence has been 

 at the rate of 1.45 feet a century. On the contrary from 1853 



1 Matthew, G. F. Post-glacial Faults at St John, New Brunswick. Am. 

 Jour. Sci. ser. 3. 1894. 48:501-3. 



2 See bibliography, No. 148a. 



