794 



SCIENCE. 



[N. S. Vol. XII. No. 308. 



the bar of the southwest pass advanced 

 between the years 1838 and 1874 a distance 

 of over two miles, covering an area 2.2 miles 

 in width with a deposit of sediment 80 feet 

 in thickness ; outside the bar, where the 

 sea is 250 feet in depth, sediment accumu- 

 lates, according to Messrs. Humphreys and 

 Abbot, at a rate of two feet yearly. It is 

 quite possible, indeed it is very likely, that 

 some of our ancient strata bave been formed 

 with corresponding rapidity. ISTo gravel of 

 coarse sand is deposited over the Missis- 

 sippi delta ; such material is not carried 

 further seawards than New Orleans. Thus 

 the vast sheets of conglomerate and sand- 

 stone which contribute so largely to some ' 

 of our ancient systems, such as the Cam- 

 brian, Old Red Sandstone, Millstone Grit, 

 and Coal Measures, must have accumulated 

 under very different conditions, conditions 

 for which it is not easy to iind a parallel ; 

 but ia any case these deposits afford evi- 

 dence of very rapid accumulation. 



These considerations will not tempt us, 

 however, to modify our estimate of one foot 

 in a century; for though in some cases this 

 rate may have been exceeded, in others it 

 may not have been nearly attained. 



Closely connected with the rate of depo- 

 sition is that of the changing level of land 

 and sea ; in some cases, as in the Wealden 

 delta, subsidence 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 aver- 

 age rate at which movements of the ground 

 are proceeding ; it might naturally be ex- 

 pected that the accurate records kept by 

 tidal gauges in various parts of the world 

 would afford us some information on this 

 subjitcfc ; 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 im- 

 mediate application are the results of Herr 

 L. Holmstrom's observations in Scandina- 

 via, which prove an average rise of the pen- 

 insula at the rate of three feet in a century 

 to be still in progress ; and Mr. G. K. Gil- 

 bert's measurements in the Great Lake dis- 

 trict of North Amei-ica, which indicate a 

 tilting of the continent at the rate of three 

 inches per hundred 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 observa- 

 tions have accumulated and the disturbing 

 effect of the inconstancy of the sea level 

 eliminated. 



It may be objected that in framing our 

 estimate we have taken into account me- 

 chanical sediments only, and ignored others 

 of equal importance, 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 aston- 

 ishing rate of six feet in height per annum. 



In respect of coal there is much to sug- 

 gest that its growth was rapid. The car- 

 boniferous period well deserves its name, 

 for never before, never since, have Carbona- 

 ceous deposits accumulated to such a re- 

 markable thickness or over such wide areas 

 of the earth's surface. The explanation is 

 doubtless partly to be found in favorable 

 climatal conditions, but also, I think, in the 

 youthful energy of a new and overmaster- 

 ing type of vegetation, which then for the 

 first time acquired the dominion of the 

 land. If we turn to our modern peat- 

 bogs, the only Carbonaceous growths avail- 

 able for comparison, we find from data given 

 by Sir A. Geikie that a fairly average rate 



