GAPS IN THE GEOLOGICAL BECORD. 167 



Iii reality this ideal continuous series of strata is interrupted by 

 numerous and often large gaps, which determine the petrographical 

 and palseontological differences, often strongly marked, between 

 successive strata, and correspond to periods of inactivity, or, as 

 may happen, to periods when the results of sedimentary action 

 have been again destroyed. These interruptions of local deposits 

 are explained by the constant alterations of level which the 

 surface of the earth has undergone in every period in consequence 

 of the reaction of the molten contents of the earth against its firm 

 crust. 



As we see in the present time that wide tracts of country are 

 gradually sinking (west coast of Greenland, coral islands), while 

 others are being slowly elevated (west coast of South America, 

 Sweden) ; that strips of coast line are suddenly submerged beneath 

 the sea by subterranean forces, and that islands as suddenly appear; 

 so it was in earlier periods. Elevation and depression were at work, 

 perhaps uninterruptedly, causing a gradual, more rarely a sudden 

 (and then locally confined) interchange between land and sea. 

 Basins of the sea rising with gradual movement became dry land and 

 rose up first as islands, and afterwards as connected continents, the 

 different deposits of which, with their included fossils, bear witness 

 of the sea which once covered them. On the other hand, great 

 continents sank beneath the sea, leaving perhaps their highest moun- 

 tain peaks appearing as islands, and again became the seat of fresh 

 deposition of strata. In the first case there would be an interruption 

 of deposit, while in the latter there would result, after a longer or 

 shorter period of inactivity, the beginning of a new formation. Since, 

 however, elevations and depressions, even though affecting districts of 

 great extent, must always be locally confined, the commencement and 

 interruption of formations of equal age have not taken place every- 

 where at the same time. Deposits continued a long time on one tract 

 after they had ceased on another; hence the upper and lower boun- 

 dary of equivalent formations may show great want of uniformity, 

 according to the different locality. This explains how it is that for- 

 mations lying one above the other are composed of strata of very 

 variable thickness, and why we can only in rare cases supply the gaps 

 in the series of these strata from strata found in other countries. 

 The whole succession of formations known to us up to the present 

 time is not sufficiently complete to form an entire and uninterrupted 

 series of the sedimentary formations. There are still numerous and 

 important gaps in the geological record which we may expect to 



