352 POPULAR SCIENCE MONTHLY. 



fusion with the tidal and annual because its period is so much longer, 

 being more than 2,000 times that 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 practical geolo- 

 gist would never confuse the deposit occasioned by a single storm, for 

 example, 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 dis- 

 criminate the precessional from these it is necessary to employ other 

 characters. Such characters 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 between 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, known as yet only in the field of 

 hypothesis, is hypothetic-ally a running-down oscillation, like the lessen- 

 ing sway of the cradle when the push is no longer given. 



But the precessional motion pulses steadily 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 conditions are favorable and 

 the connection is made, the record should reflect the persistence and the 

 regularity of the inciting rhythm. 



The search of the rocks for records of the ticks of the precessional 

 clock is an out-of-door work. Pursued as a closet study it could have 

 no satisfactory outcome, because the printed descriptions of rock se- 

 quences are not sufficiently complete for the purpose; and the closet 

 study of geology is peculiarly exposed to the perils of hobby-riding. A 

 student of the time problem cannot be sure of a persistent, equable 

 sedimentary rhythm without direct observation of the characters of the 

 repeated layers. He needs to avail himself of every opportunity to study 

 the series in its horizontal extent, and he should view the local problem 



