52 WARREN UPHAM^ M.A._, D.SC, F.G.S.A., ON THE SAN FRANCISCO 



1886, a recoided number of 1,512 shocks, or an average of 

 about thirty-five yearly. " It is the Chilian coast and Andes," 

 remarks Button, " in which the seismicity of South America 

 reaches its highest development." 



Secular Movements of the Earth's Crust. 



Darwin, in his Geological Ohservations on South America, 

 noted proofs, in raised shore lines with recent marine shells, 

 that a vast area of this Pacific coast, extending from Southern 

 Patagonia to Lima, a distance of about 2,500 miles, had been 

 lately much uplifted, the vertical measure of the movement 

 varying from 85 to 1,300 feet. The maximum uplift observed 

 was in the vicinity of Valparaiso, where, however, in the 

 220 years preceding 1834 the vertical upward movement had 

 not exceeded nineteen feet ; but during the relatively short 

 time of seventeen years, from 1817 to 1834, it had amounted 

 to ten feet and seemed still in progress. Only a part of that 

 average uprise of more than half a foot yearly could be 

 attributed to a violent earthquake which occurred in 1822, 

 and the remainder had taken place at a very slow and 

 imperceptible rate. 



It may be thought that the whole uplift of the Chile coast 

 was accompanied by faulting and slight earthquakes, as more 

 than thirty perceptible shocks have been observed on an 

 average yearly in that district. But it seems more probable 

 that in some other regions, and therefore even to some degree 

 in Chile, extensive secular uplifts or depressions of large 

 areas, continuing through several or many centuries, have 

 taken place by gentle and moderate flexure of tlie rock crust, 

 without faulting, or at least without any large displacement, 

 and perhaps without energetic earthquakes. 



Such a gradual crustal movement appears to have elevated 

 the area of the Glacial Lake Agassiz, during the recession of 

 the ice-sheet at the end of the Glacial period, where now is 

 the basin of the Eed Kiver and Lake Winnipeg. This area, 

 500 miles long or more from south to nortli, and measuring 

 50 to 150 miles or more in width, was differentially uplifted 

 to a maximum vertical amount of probably about 500 feet in 

 tlie geologically short time, estimated about 1,000 years, while 

 the glacial lake existed ; and the uplift was nearly or (piite 

 finished before the latest remnant of the ice-sheet on Central 

 Canada was melted away, thereby reducing Lake Agassiz to 

 its present representatives, lakes Winnipeg and Manitoba. 

 The continuousness and the gradual and unbroken northward 



