224 THE POPULAR SCIENCE MONTHLY. 



According to the best established and prevailing views, the 

 great sea-beds, the deep basins, are the original features of the 

 "face of the earth." They are the first depressions that were 

 made after the planet's surface was solidified and its structure 

 began to shrink from cooling. These depressions have probably 

 been increasing vertically through all the geological ages, and 

 therein lies the cause of the constant increase of the land to the 

 present time. But the constancy of this increase is a fact only as 

 a whole ; for there have been times when the sea rose over the 

 shore and overflowed a large part of the land. The last great 

 transgression of this kind occurred at the beginning of the Mio- 

 cene Tertiary, when extensive tracts of the Old World were cov- 

 ered with water. Parts of Italy, Portugal, southern France, north- 

 ern Switzerland, southern and northern Germany, the Vienna 

 basin, and the Hungarian plain, and of the lowlands of eastern 

 Europe and the Black Sea region away into the interior of Asia 

 to Persia, with the plains of North Africa still bear the marine 

 deposits of that period, with the remains of organisms, mostly of 

 extinct species, that inhabited the adjoining seas. The waters re- 

 tired to their beds during the Pliocene, and the present boundary- 

 lines of the land regions, aside from a few erosions, were shaped 

 during the same period. Similar processes took place repeatedly 

 in the earlier periods of the earth's growth, and to them are as- 

 cribed most of the changes that have taken place in the species of 

 animals and plants ; for these encroachments of the sea forced the 

 living world into a narrower space, and entailed a fierce struggle 

 for existence, in which the less valiant species succumbed. The 

 retreat of the sea again permitted a fruitful development of life 

 and the origination of new species. 



An important circumstance has been brought to light in the 

 investigation of the fluctuations of the ocean. The continents 

 have been overflowed several times. Suess, who has made the 

 most thorough study of the subject, has recognized six principal 

 periods of submersion, and as many of dryness. But no indubi- 

 tably deep-sea deposits later than the Silurian are to be found on 

 the present continents. The great sea-beds are primitive; and 

 primitive likewise are the socles of the continents, standing as 

 equally sturdy champions with the briny flood, sometimes in the 

 alternations of the contest lying under it, overcome by the sea, at 

 other times shaking it off and sending it back within its lines. 



The cause of these processes is still awaiting explanation. Cel- 

 sius and Linnaeus, who observed the gain of land on the Baltic 

 coast during the last century, expressed the opinion that the sea 

 was retreating. This view was contested at the beginning of the 

 present century by Leopold von Buch, who thought that Scandi- 

 navia was rising. Lyell and Darwin advanced the theory of the 



