202 C. Lloyd Morgan — Geological Time. 



data on which to base any calculations. But I see no reason why 

 these formations should not accumulate as rapidly as those in a delta. 

 We will therefore take, hypothetically, one-tenth of an inch as the 

 average rate of the accumulation of all this series of deposits, 



IJ. In considering the rate of accumulation over such an area as 

 the North Sea, for example, we find but little on which to base any 

 calculations. Some of the rivers which fall into that sea, the Rhine 

 and Scheldt, for instance, form great alluvial flats, while the finer 

 and coarser materials brought down by other rivers are caught up by 

 the tides which sweep by their mouths, and become far more widely 

 scattered. The rate of deposit will be, then, in different parts of 

 the area, very variable. But if we take the area of country which 

 drains into the German Ocean at twice the size of that sea, and the 

 rate of subaerial denudation to be one foot in three thousand years, 

 we shall find that the average rate of deposit over the German Ocean 

 is -r\-5 of an inch per annum. 



Along the coast-line of a great continent the rate at which sedi- 

 mentary deposits are formed will be still slower. A glance at a 

 map of North America will show from how vast an area of that 

 continent the Mississippi collects its waters. We have seen that 

 the sediment borne by those waters is collected over a comparatively 

 small area, in the neighbourhood of the Gulf of Mexico. Along 

 all the eastern coast, from Labrador to Florida Straits, but little 

 sedimentary deposit is being formed. If we take the strip of land 

 which is drained into this portion of the Atlantic to be, on the 

 average, two hundred and fifty miles broad, and the area over which 

 deposit is taking place to be a strip extending for a hundred miles 

 from the coast-line, and the rate of denudation to be the same as in 

 the Mississippi basin, we find that the rate of accumulation is -2^0- 

 of an inch per annum. If again we turn to South America, we find 

 that almost the entire drainage of the continent is eastwards. 

 Taking the area which drains in this direction at about five and 

 a quarter million square miles, the rate of denudation the same as 

 in the Mississippi basin, soVo of a foot per annum, the eastern 

 coast-line at about 8500 miles in length, and the zone of deposit 

 to stretch one hundred miles from the shore-line, we shall find that 

 in this area about -gV of an inch of rock is deposited each year. 

 We must remember that much of this sedimentary matter will be 

 collected in the neighbourhood of the four great rivers, the Amazons, 

 La Plata, Orinoco, and San Francisco. Altogether, then, we may 

 consider that the rate of formation of mechanical deposits at a con- 

 siderable distance from the principal foci — as we may call the delta, 

 or estuarine areas — is not greater than xo~oj or perhaps, 20-0 of an 

 inch per annum. 



III. Professor Huxley, in his article on " Coral and Coral Eeefs," 

 quotes the following estimate of the rate of Coral deposit from Dana's 

 Manual of Geology. " The rate of growth of the common branching 

 Madrepore is not over one and a half inches a year. As the 

 branches are open, this would not be equivalent to more than half 

 an inch of solid coral for the whole surface covered by the Madre- 



