240 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



know little of its geologic effect, and, moreover, it is a complex cycle 

 built up from the cycles of the motions of the various planets, which 

 also affect the sun and, through the sunspot cycle, the earth. 



Thus all these cycles are as yet unsuitable either as units, or, we 

 may add, to be counted in estimating geologic time. We say "as 

 yet," for the cyclothcms of R. C. Moore ^ in the coal measures, the 

 rhythmic bandings of flint in the chalk of the Norman coast, and 

 similar rhythms in many other places suggest possibilities. 



There is still another class of cycles deserving a word in this place, 

 since they are highly important in geology, oven though not suited 

 for time units. It has been suggested that there is a steady and pro- 

 gressive accumulation of strain in the crust of the earth, to wliich it 

 yields, perhaps spasmodically, or perhaps periodically. One might 

 consider these as unit cycles. As wo have already remarked, this is 

 quite conceivable. Not only the discharge of many geysers, but the 

 action of the violin bow and of the water clock is of this nature. 

 But when we examine a little more closely we see that at present, 

 at least, if we take the table given by Stillc, figure 2 (after Bull. Amer. 

 Assoc. Petroleum Gool., vol. 20, p. 852, July 193G), such cycles 

 cannot be of use in measuring geologic time. The recognition of 

 such cycles, and the proof of their regularity, if such there be, is 

 rather a goal of geologic investigation. 



There are two causes of the accumulating strain which have been 

 cited. The one is tlic shrinkage of the interior due to the loss of 

 energy, especially as heat, causing the crust to tend to collapse when 

 the strain thus produced surpasses its strength. This idea of the crust 

 wrinkling like an old apple has been accepted by a line of writers of 

 distinction, the present champion being Prof. H. Stille,^ of Gottingen. 

 That there has been a cycle of sedimentation corresponding to each 

 great geologic period is widely accepted and goes back at least to 

 Newberry. The concept has been most systematically worked out 

 by Schuchert,*° who also believes that the periods are separated by 

 epochs of wide crustal disturbance. That these are relatively short 

 and sharp is also widely held, as has been recently emphasized by 

 R. T. Chamberlin." But this is not universally accepted, and the 

 idea has been vigorously opposed by E. W. Berry." 



In view of the facts that the volcanoes of Italy and Iceland and 

 other regions have been pretty continuously active since some time 

 in the Tertiary, which we shall find reason to believe was millions of 

 years ago, and that mountains like the Coast Range of California 



' studies on tho Carboniferous of the midcontinent region, Bull. Araer. Assoc. Petroleum Oeol., vol. 13, 

 p. 191, 1929. 

 • Bull. Amer. Assoc. Petroleum Geol., vol. 20, pp. 849-8S0, 1936. 



•o Nat. Res. Counc. Bulls. 80 and 85. See also Bull. Oeol. Soc. Amor., vol. 32, pp. 147-160, 1921. 

 " Certain aspects of geological classifications and correlations. Science, vol. 81, p. 184, 1935. 

 " Shall we return to cataclysmic Rcolopiy? Amer. Journ. Scl., ser. 6, vol. 17, pp. 1-12, January 1920. 



