Juke 29, 1900.] 



SCIENCE. 



1011 



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 

 significant 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 dis- 

 tributed, 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 inter- 

 vals can be rationally bridged over by the 

 aid of time ratios. 



For this purpose there is only one of 

 the imposed rhythms of practical value, 

 namely, the processional ; 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 accentu- 

 ation of the precessional, can not ordinarily 

 be distinguished from it. But none of these 

 qualifications apply to the precessional. It 

 is not liable to confusion with the tidal and 

 annual because its period is so much longer, 

 being more than 2000 timesrthat of the 

 annual. It has an eminently practical and 

 convenient magnitude, in that its physical 

 manifestation is well above the microscopic 

 plane, and yet not so large as to prevent 

 the frequent bringing of several examples 

 into a single view. It is also practically 

 regular in period, rarely deviating from 



the average length by more than the tenth 

 part. 



From the greater number of original 

 rhythms it is distinguished, just as from the 

 annual and tidal, by magnitude. The prac- 

 tical geologist would never confuse the de- 

 posit occasioned by a single storm, for exam- 

 ple, with the sediments accumulated during 

 an astronomic cycle of 20,000 years. But 

 there are other original rhythms, known or 

 surmised, which might have magnitudes of 

 the same general order, and to discriminate 

 the precessional from these it is necessary 

 to employ other characters. Such charac- 

 ters are found in its regularity or evenness 

 of period, and in its practical perpetuity. 

 The diversion of the mouth of a great river 

 such as the Hoang Ho or the Mississippi, 

 might recur only after long intervals ; but 

 from what we know of the behavior of 

 smaller streams we may be sure that such 

 events would be very irregular in time as 

 well as in other ways. The intervals be- 

 tween volcanic eruptions at a particular 

 vent or in a particular district may at times 

 amount to thousands of years, but their 

 irregularity is a characteristic feature. The 

 same is true of the recurrent uplifts by 

 which mountains grow, so far as we may 

 judge them by the related phenomena of 

 earthquakes ; and the same category would 

 seem to hold also the theoretically recur- 

 rent collapse of the globe under the strains 

 arising from the slowing of rotation. The 

 carbon-dioxide rhythm, know as yet only 

 in the field of hypothesis, is hypothetically 

 a running-down oscillation, like the lessen- 

 ing sway of the cradle when the push is no 

 longer given. 



But the precessional motion pulses stead- 

 ily on through the ages, like the swing of a 

 frictionless pendulum. Its throb may or 

 may not be caught by the geologic process 

 which obtains in a particular province and 

 in a particular era, but whenever the con- 

 ditions are favorable and the connection is 



