214 CLASSICS OF MODERN SCIENCE 

 survey of the tertiary and modern strata, which constitute a more com- 

 plete and unbroken series than rocks of older date, that the extinction 

 and creation of species have been, and are, the result of a slow and 

 gradual change in the organic world. 



Uniformity of change considered, thirdly, in reference to subter- 

 ranean movements. — Thirdly, to pass on to the last of the three topics 

 before proposed for discussion, the reader will find, in the account 

 given in the Second Book, Vol. II., of the earthquakes recorded in 

 history, that certain countries have, from time immemorial, been 

 rudely shaken again and again ; while others, comprising by far the 

 largest part of the globe, have remained to all appearance motionless. 

 In the regions of convulsion rocks have been rent asunder, the surface 

 has been forced up Into ridges, chasms have opened, or the ground 

 throughout large spaces has been permanently lifted up above or let 

 down below its former level. In the regions of tranquillity some 

 areas have remained at rest, but others have been ascertained, by a 

 comparison of measurements made at different periods, to have arisen 

 by an insensible motion, as in Sweden, or to have subsided very 

 slowly, as in Greenland. That these same movements, whether as- 

 cending or descending, have continued for ages in the same direction 

 has been established by historical or geological evidence. Thus we 

 iind on the opposite coasts of Sweden that brackish water deposits, 

 like those now forming in the Baltic, occur on the eastern side, and 

 upraised strata filled with purely marine shells, now proper to the 

 ocean, on the western coast. Both of these have been lifted up to an 

 elevation of several hundred feet above high-water mark. The rise 

 within the historical period has not amounted to many yards, but the 

 greater extent of antecedent upheaval is proved by the occurrence in 

 inland spots, several hundred feet high, of deposits filled with fossil 

 shells of species now living either in the ocean or the Baltic. 



It must in general be more difficult to detect proofs of slow and 

 gradual subsidence than of elevation, but the theory which accounts 

 for the form of circular coral reefs and lagoon islands, and which will 

 be explained in the concluding chapter of this work, will satisfy the 

 reader that there are spaces on the globe, several thousand miles in 

 circumference, throughout which the downward movement has pre- 

 dominated for ages, and yet the land has never, in a single instance, 

 gone down suddenly for several hundred feet at once. Yet geology 



