Page:Philosophical Transactions - Volume 145.djvu/122

106, 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 progression.

Although it be thus highly improbable that we can ever form an approximate estimate 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 connecting historical and geological time ?

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 historical period to afford a measure of the time occupied in the production of the antecedent 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 since the change in the form and structure of the land under examination first began.

Of the various agencies which modify the earth's surface, rivers are the most constant, the most uniform in their operation within given periods, and the most appreciable 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 such 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,