H. W. Pearson — Oscillations of Sea-level. 229 



From the argument preceding it seems necessary to conclude that 

 in future study of changes in the sea-level, discrimination must be 

 made between each of the following causes which may afifect the 

 oceanic borders : — 



1. Seek the effects produced by the bodily transference of water 

 to and from the north. These effects would be universal over one 

 hemisphere. 



2. Detect and eliminate those movements of upheaval or depression 

 due to variation in flow of ocean currents under the operation of 

 Ferrell's law. These effects are local in their nature. 



3. In deltas always suspect that any observed depression may be 

 due to a local settlement of the ground itself, and such data there 

 gathered may offer no ai'gument whatever in favour of a rise in 

 sea-level. 



4. Eliminate and avoid such coastal changes as may be due to 

 erosion of or accretion to shore-lines. In changes of this, century 

 it is generally possible to do this. In changes that have occurred 

 in the distant past we shall find much difficulty in separating results 

 of erosion or accretion from the resvdts of real changes in the 

 sea-level ; therefore, in past ages much testimony will be found 

 accumulated against us which our analysis will be unable to 

 remove; we must expect, therefore, many of these apparently con- 

 flicting observations. 



All the evidence discussed hereto has been gathered on the oceanic 

 coastlines ; these data, as we have seen, testify to a recent protrusion 

 of the entire north, and that this apparent vertical uplift increased 

 in amount as we approach the Pole. There is, however, evidence in 

 existence, obtained from our great lakes, showing the same law of 

 greater elevation to the north. 



Mr. G. K. Gilbert, in the 18th Ann. Rep. U.S. Geol. Survey, has 

 shown that this excess of upheaval at the north has been of recent 

 occurrence in the interior of our continent. A careful study of the 

 changes in level, during the present century, of the great lakes 

 enables Gilbert to announce this law with certainty. 



With his inferential conclusions, however, in the light of our own 

 investigation, we are compelled to differ. He assumes that this 

 motion may continue indefinitely, and if so, he shows that in time 

 the Niagara Falls will cease their flow, and a new outlet to the great 

 lakes be placed in operation near Chicago. This result he reaches 

 in a logical manner from the data examined, but we see that 

 observations reaching back only one hundred years allow us to form 

 no certain opinion as to whether this motion will continue indefinitely 

 in one direction or otherwise. Then, again, the area of investigation 

 was of too limited a character. We have seen that to obtain the law 

 governing these risings and sinkings, it is not only necessary to 

 study at one field of view the entire Northern Hemisphere, but to 

 carry our investigation back in time as far as history or tradition 

 will allow. When this has been done we see that Gilbert's observed 

 changes in level fall into line as part and parcel of one complete 

 system, universal over the entire North. 



