38 THE AGE OF THE EARTH 



seawards, then twice the average would give us the maxi- 

 mum rate of deposition : this would be one foot in 120 

 years. But the sheets of deposited sediment are not 

 merely thicker towards the land, thinner towards the sea, 

 they also increase in thickness towards the rivers in which 

 they have their source, so that a very obtuse-angled cone, 

 or, better, the down-turned bowl of a spoon, would more 

 nearly represent their form. This form tends to dis- 

 appear under the action of waves and currents, but a 

 limit is set to this disturbing influence by the subsidence 

 which marks the region opposite the mouth of a large 

 river. By this the strata are gradually let downwards, so 

 that they come to assume the form of the bowl of a spoon 

 turned upwards. Thus a further correction is necessary 

 if we are to arrive at a fair estimate of the maximum rate 

 of deposition. Considering the very rapid rate at which 

 our ancient systems diminish in thickness when traced in 

 all directions from the localities where they attain their 

 maximum, it would appear that this correction must be a 

 large one. If we reduce our already corrected estimate 

 by one-fifth, we arrive at a rate of one foot of sediment 

 deposited in a century. 



No doubt this value is often exceeded ; thus in the case 

 of the Mississippi River the bar of the south-west 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 

 accumulates, according to Messrs. Humphreys and Abbot, 

 at a rate of 2 feet yearly. It is quite possible, indeed it 

 is very likely, that some of our ancient strata have been 

 formed with corresponding rapidity. No gravel nor 

 coarse sand is deposited over the Mississippi delta ; such 

 material is not carried further seawards than New 

 Orleans. Thus the vast sheets of conglomerate and 



