i 



81 



capable of sustaining life of any kind at all resembling what we now find upon it, 

 all the reply we can obtain is, that the period would be so enormous that any 

 definite answer would be mere guesswork, and that it is a period which can only 

 be spoken of b}- hundreds of millions of j'ears. If then, such is the estimate of 

 the period of formation of this earth when, as I have said, we are led to believe 

 the processes of nature were far more rapid, what shall we say of the period of 

 time which has elapsed since the formation of the earth, and during which it has 

 been passing through those changes which are termed its geological development? 

 To answer this question definitely, I fear that astronomy will help us as little as 

 before, but it does teach us to recognise the vastness of the period through which 

 our earth has passed, and the impossibility of dogmatically laying dowa any even 

 apiproximate time for its existence, and it also teaches us that, although vast and 

 immeasurable as the time may have been, to our ideas at least, of the growth 

 and existence of the planets, and of the yet unexhausted time which they have to 

 endure ; yet, almost to a certainty, this earth, with the other members of the 

 solar system, had their birth, and will have their destruction in time ; and, that 

 viewed in relation to the other systems found in the heavens, our system only 

 occupies a mere speck in space, and only a moment in time. Leaving, therefore, 

 astronomy as an insufficient guide to lus in this research, let us come now to the 

 consideration of what at first sight appear to be somewhat safe and trustworthy 

 guides in estimating geological periods, namely, to the consideration of the pro- 

 cesses of deposition of sedimentary rocks, and of denudation of earth's surface. 

 These processes, when they can be accurately observed, do, no doubt, give us 

 some of the best means of estimating geological time ; as, when it has been deter- 

 mined by observation that a certain amount of deposition of mud or sand is being 

 laid over any particular surface, such as the delta of any large river, in a "iven 

 time, it can be inferred, by measuring the depth of that deposit, how long a 

 period of time it has taken to make that deposit ; and hence, how long a period 

 of time similar conditions of land and water to those now existing have ob- 

 tained. For instance, we may take one of the best known, and perhaps the 

 simplest, examples of such deposition — the river Nile— and see what sort of infer- 

 ence can be drawn from its operations. It has been observed that the Nile, by 

 its annual overflow, deposits every century about three-and-a-half inches of mud 

 and it has also been found that that mud deposit at Memphis extends to a depth 

 of forty feet ; and, therefore, if we take for granted that the action of the river 

 has been uniform, we draw the inference that the Nile mud has been about 

 13,700 years in the process of formation. Similar observations can be made, and 

 similar conclusions can be arrived at, by taking any of the other great rivers 

 but for our short paper one example must suflBce. It must be now my part to 

 try and show how very uncertain are our premises in such cases of deposition 

 and, therefore, how uncertain and untrustworthy must be our conclusions. And 

 first I must draw your attention to some interesting and important facts con- 

 nected with the powers of running water. We learn from the researches of Mr. 

 W. Hopkins that " the power of water to move bodies that are in it, increases as 

 the sixth power of the velocity of the current." That is, if we double the velo- 



