286 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



it is only a century since measurements of height have been 

 taken in sufficient number and with sufficient accuracy to 

 afford secure data for comparison. For geological processes a 

 hundred years is a period of as small significance as a single 

 second in the history of mankind. 



It is easier to determine variations of level at sea-coasts, 

 but even there it is often doubtful whether a change of 

 relative level is due to displacements of the land or of the 

 ocean ; and the observer has to be careful not to mistake for 

 secular movements any of the effects of sedimentation in 

 heightening the land, or of marine erosion and subaerial 

 denudation in breaking down the coast. The occurrence of 

 submerged forests and beds of peat, old roads and other 

 human structures on the sea-floor are among the more secure 

 evidences of a depression of the land or uprise of the water. 

 On the other hand, remains of harbour and pier con- 

 structions, and fragments of vessels found at a height above 

 the existing sea-level, or at some distance inland, give evidence 

 of a secular movement of land-elevation or retreat of the sea 

 within historic ages. Former coast-lines and terraces can 

 sometimes be. identified many hundred feet above the present 

 surface of the ocean. The exposure of delta deposits is 

 usually regarded as a sign of land elevation, whereas long 

 narrow fiords occurring as the continuation of river-valleys 

 towards the sea, are regarded as proofs that a coast is 

 undergoing subsidence. 



The oldest direct observations on relative changes of level 

 were made in Scandinavia. Hjarne observed in 1702 that the 

 Swedish coasts were frequently extended in consequence of a 

 retreat of the sea, and Celsius and Linnaeus afterwards made 

 investigations on the rate of retreat by means of boundaries 

 and marks on the rocks at Gefle and Kalmar. Celsius in 1 743 

 read his memorable paper at the Swedish Academy of Science, 

 in which he argued that the volume of water in the ocean was 

 diminishing. He calculated the sinking of the ocean surface- 

 level at forty-five inches in a century. Linnaeus supported 

 the views of Celsius, but Bishop Browallius (1756), E. D. 

 Runeberg, and the Danish scientist, Jessen (1763), opposed 

 them. E. D. Runeberg argued that the changes on the 

 Swedish coast were due to elevation of the land in consequence 

 of earthquakes. 



The Scottish mathematician, Professor Play fair, in 1802 



