312 EVOLUTIONAL GEOLOGY. 



where the sea is 250 feet in depth, sediment accumulates, according to 

 Messrs. Humphre3^s and Abbot, at a rate of 2 feet 3'earh\ It is quite 

 possible, indeed it is very likely, that some of our ancient strata have 

 been formed with corresponding- rapidity. No gravel or coarse sand 

 is deposited over the Mississippi delta. Such material is not carried 

 farther seaward than New Orleans. Thus the vast sheets of conglom- 

 erate and sandstone which contribute so largely to some of our ancient 

 systems, such as the Cambrian, Old Red Sandstone, Millstone Grit, 

 and Coal Measures, must have accumulated under very different con- 

 ditions, conditions for which it is not easy to find a parallel; but in 

 any case these deposits afford evidence of very rapid accumulation. 



These considerations will not tempt us, however, to modify our 

 estimate of 1 foot in a centurj'^; for though in some cases this rate 

 may have been exceeded, in others it ma}' not have been nearly 

 attained. 



Closely connected with the rate of deposition is that of the changing 

 level of land and sea; in some cases, as in the Wealden delta, subsid- 

 ence and deposition appear to have proceeded with equal steps, so that 

 we might regard them as transposable terms. . It would, therefore, 

 prove of great assistance if we could determine the average rate at 

 which movements of the ground are proceeding; it might naturally be 

 expected that the accurate records kept by tidal gauges in various 

 parts of the world would afford us some information on this subject; 

 and no doubt they would, were it not for the singular misbehavior of 

 the sea, which does not maintain a constant level, its fluctuations being 

 due, according to Professor Darwin, to the irregular melting of ice in 

 the polar regions. Of more immediate application are the results of 

 Herr L. Holmstrom\s observations in Scandinavia, which prove an 

 average rise of the peninsula at the rate of 3 feet in a century to be 

 still in progress; and Mr. G. K. Gilbert's measurements in the Great 

 Lake district of North America, which indicate a tilting of the conti- 

 nent at the rate of 3 inches per 100 miles per century. But while 

 measurements like these may furnish us with some notion of the sort 

 of speed of these changes, they are not sufficient even to suggest an 

 average; for this we must be content to wait till sufficient tidal obser- 

 vations have accumulated, and the disturbing effect of the inconstancy 

 of the sea level is eliminated. 



It may be objected that in framing our estimate we have taken into 

 account mechanical sediments only, and ignored others of equal impor- 

 tance, such as limestone and coal. With regard to limestone, its thick- 

 ness in regions where systems attain their maximum may be taken as 

 negligible; nor is the formation of limestone necessarily a slow process. 

 The successful experiments of Dr. Allan, cited by Darwin, prove that 

 reef-building corals may grow at the astonishing rate of 6 feet in height 

 per annum. 



