52 



SCIENCE-GOSSIP. 



illustration of this may best be exemplified by a 

 modern event, such as the great Krakatoa eruption 

 in 1883, when the mud deposits in Java were 

 correlated in time with great collections of pumice 

 stone on the shores of Africa, as also with deposits 

 of fine dust derived from the same store of volcanic 

 matter, and showered down over a very large area. 

 There are other intermittent phenomena which 

 have been traced, although often of a somewhat 

 prolonged and uncertain duration. One group of 

 these consists in the elevation of one area and the 

 subsidence of a contiguous one as parts of the same 

 general earth movement. A second group consists 

 in the beds of one area being folded and in such 

 folding shifting about over large areas. The age 

 of the folding can often be determined so as to 

 enable us to sharply separate the beds of a later 

 age over the whole area that has been compressed. 

 A third group consists in the somewhat sudden 

 spread of one species in a genus of animals over a 

 large portion of the world, whereby it is possible 

 to roughly synchronize certain strata at localities 

 very distant from one another. 



Many other cases could be noticed, but it is 

 unnecessary to do so here, because the object is 

 simply to urge the desirability and possibility of 

 estimating the geological age in more ways than one. 



It is obvious that a series of independent 

 erroneous criteria can never lead to one coincident 

 conclusion ; hence, if the independent tests applied 

 do converge towards a common centre, either the 

 criteria are trustworthy, or else their diverging 

 errors in excess and in defect just counter- 

 balance each other. As I have obtained an un- 

 expected degree of coincidence, I have ventured to 

 use the scales proposed as chronological tests and 

 as time valuers. 



For the purpose of applj-ing them tentatively 

 and roughly, three simple assumptions are made : 



1. Geological time has been continuous; it is 

 representable by one million, and is divisible into 

 one million equal parts. 



2. The known strata constitute a part, or 

 possibly nearly the whole, of a geological con- 

 tinuous succession of strata. This succession is 

 representable by one million, and is divisible into 

 one million equal parts. 



3. Animal organisms form a continuous chain 

 of organisation. It is representable by one million, 

 and is divisible into one million equal parts. 



By combining these three scales, one general 

 scale is formed ; the base line is the time scale, 

 the top line is the stratigraphic scale, and the line 

 on either side is the organic grade scale for animals. 



The next step is to correlate the equal divisions 

 corresponding to time, the strata, and animals. 

 Different kinds of strata are manifestly deposited 

 at very different rates, so that one hundred 

 feet of limestone does not represent the same 



time value as one hundred feet of conglomerate. 

 Hence it is necessary to reduce the scale for 

 each kind of deposit to one scale, representing 

 the average of all. I believe a fine clay fairly 

 represents the average ; and, as rocks of the 

 nature of clay are more abundant than any 

 other, are frequently of considerable thickness, and 

 are liable to but little w-aste by solution, I adopt 

 the clay scale as the average. In this scale, the 

 thickness of a geologically continuous column is 

 calculated to be 200,000 feet, and the mass 400 

 million cubic miles. The present rate of denudation 

 is estimated at two cubic miles a year ; so that, if 

 this be taken to be the average rate (as I think may 

 safely be done, seeing that it harmonizes better 

 with observed facts than any other I have tried) the 

 inferred time value is 200 million years. Each of 

 the thousand-feet divisions of clay, therefore, repre- 

 sents one million years, while each of the million 

 divisions in the time scale equals 200 years, and 

 corresponds to two feet of clay. In order to avoid 

 the inconvenience that may arise from having 

 erroneous definite values, I propose to call the 

 period required for the deposition of 1,000 feet of 

 clay an eon. Hence generally this term has an 

 indefinite value ; yet each individual may assign 

 to it his own special definite value. In this way 

 the stratigraphic grade is adapted for all opinions. 

 In this paper I give it my own value as expressed 

 above. 



The next step is to correlate the names given to 

 strata with the divisions of the stratigraphic grade. 

 These names are often a source of bewildering 

 confusion owing to the same name being given to 

 different ranges of strata, as has, for instance, been 

 the case with the term " Cambrian." On the 

 stratigraphic grade here adopted the terms are 

 assigned a specified value whenever I can do so, 

 but such terms are intended to apply to the time 

 represented by the rocks, not to the rocks them- 

 selves nor to the groupings of fossils in them ; that 

 is, the Cambrian strata and the groupings of fossils 

 in them are coincident with the Cambrian period 

 on the typical area only. Beyond this area the 

 coincidence may be partial only, as it is quite 

 conceivable that there may be a grouping of fossils 

 like that observed in the Cambrian typical area on 

 some other area in the Silurian period of time ; but 

 by the present method of relying too exclusively 

 on fossils as the true test of age it is impossible to 

 prove this. 



Lastly, the animal groups have to be allocated 

 according to their presumed rank on the organic 

 scale. The scale I have temporarily adopted was 

 formed long before I had any idea of combining it 

 with the chronological and stratigraphic scales. 

 This organic scale is admittedly open to much 

 improvement, nevertheless, it harmonises verv 

 nearly with the other scales. 



