106 
MR. HORNER ON THE ALLUVIAL LAND OF EGYPT. 
vered, because its formation has been progressive, and the several stages of its growth 
must each have been so modified by a variety of causes, irregular in their extent, 
duration, and recurrence, that there would exist no uniformity in the rate of pro- 
gression. 
Although it be thus highly improbable that we can ever form an approximate esti- 
mate in years of the age even of the most modern strata, we are not cut off from all 
hope of being able to assign an amount in years to the duration of some of the great 
geological changes which, in past ages, the present surface of the earth has undergone, 
by causes that are still in operation. It has been estimated that the delta of the 
Mississippi must have required not less than 100,000 years for its formation, and that 
the recession of the Falls of Niagara to their present position has been the work of 
many thousand years*. But even here we have probability only to rest upon, strong 
though it be ; may we not hope to arrive at the knowledge of some instances when 
our estimates may possess some degree of precision, where we may find a link con- 
necting historical and geological time-f? 
If in a country in which a certain alteration in the land has occurred, we know 
that such alteration has taken place in part within historical time, and if the entire 
change under consideration presents throughout a tolerable uniformity of character, 
shall we not be justified in holding the portion that has taken place within the histo- 
rical period to afford a measure of the time occupied in the production of the ante- 
cedent part of the same change? If a region exists where such a blending, as it were, 
of geological and historical time occurs, we may then be able to estimate in definite 
terms, the time that has elapsed sinee the change in the form and structure of the 
land under examination first begfan. 
Of the various agencies which modify the earth’s surface, rivers are the most con- 
stant, the most uniform in their operation within given periods, and the most appre- 
ciable in their effects. The materials which they transport from the higher parts of 
their course are frequently spread over an extensive surface in the lower lands near 
their mouths, and encroach upon the sea, leaving far inland towns that at one time 
stood on the shore. But when the foundations of sueh towns are on detrital travelled 
materials, they show that similar geological changes had been in progress before the 
first buildings in these towns were erected. If the date be known when such towns 
were last frequented as sea-ports, we can judge of the extent of geological change 
brought about between that period and the present time. But as the more recently 
transported materials, those which have accumulated during the historical period, 
* Sir Charlks Lyell, Second Visit to the United States, vol. ii. p. 250, and Travels in North America, 
vol. i. p. 34. 
t Strictly speaking, the present day is “ geological time for not an hour passes without the crust of the 
earth, externally and internally, undergoing a change. Meteoric forces are ever acting on the rocks, rivers are 
transporting to distant parts the loosened particles, and springs and volcanic forces are bringing up from the 
interior materials that are spread over the outer surface of the earth. 
