RHYTHMS AND GEOLOGIC TIME. 351 



eventually be established or disproved, and should they be established 

 it is possible that similar correlations may be made between events far 

 more remote. 



The studies of these several rhythms, while they have led to the 

 computation of various epochs and stages of geologic time, have not 

 yet furnished an estimate either of the entire age of the earth or of any 

 large part of it. Nevertheless, I believe that they may profitably be 

 followed with that end in view. 



The system of rock layers, great and small, constituting the record 

 of sedimentation, may be compared to the scroll of a chronograph. The 

 geologic scroll bears many separate lines, one for each district where 

 rocks are well displayed, but these are not independent, for they are 

 labeled by fossils, and by means of these labels can be arranged ir proper 

 relation. In each time line are little jogs — changes in kind of xock or 

 breaks in continuity — and these jogs record contemporary events. A 

 new mountain was uplifted, perhaps, on the neighboring continent, or 

 an old uplift received a new impulse. Through what Davis calls stream 

 piracy a river gained or lost the drainage of a tract of country. Escap- 

 ing lava threw a dam across the course of a stream, or some Krakatoa 

 strewed ashes over the land and gave the rivers a new material to work 

 on. The jogs may be faint or strong, many or few, and for long dis- 

 tances the lines may run smooth and straight; bat so long as the jogs 

 are irregular they give no clue to time Here and there, however, the 

 even line will betray a regularly recurring indentation or undulation, 

 reflecting a rhythm and possibly signiticant of a remote pendulum whose 

 rate of vibration is known. If it can be traced to such a pendulum 

 there will result a determination of the rate at which the chronograph 

 scroll moved when that part of the record was made; and a moderate 

 number of such determinations, if well distributed, will convert the 

 whole scroll into a definite time scale. 



In other words, if a sufficient number of the rhythms embodied in 

 strata can be identified with particular imposed rhythms, the rates of 

 sedimentation under different circumstances and at different times will 

 become known, and eventually so many parts of geologic time will have 

 become subject to direct calculation that the intervals can be rationally 

 bridged over by the aid of time ratios. 



For this purpose there is only one of the imposed rhythms of practi- 

 cal value, namely, the precessional; but that one is, in my judgment, of 

 high value. The tidal rhythm can not be expected to characterize any 

 thick formation. The annual is liable to confusion with a variety of 

 original rhythms, especially those connected with storms. The rhythm 

 of eccentricity, being theoretically expressed only as an accentuation of 

 the precessional, can not ordinarily be distinguished from it. But none 

 of these qualifications apply to the precessional. It is not liable to con- 



