1918] Davis: The Radiolarian Cherts of the Franciscan Group 391 



work of A. E. Doug-las, 159 who describes striking variations in the 

 rate of growth of trees. It appears that the thickness of rings of 

 growth varies according to the climatic conditions. Measurements 

 of many tree stumps have been made and the results of these measure- 

 ments compared with rainfall records over a period of forty years. 

 The result showed a very close agreement between rate of growth and 

 amount of precipitation. The measurements of old trees showed that 

 for many years back the climate has changed in a pulsatory manner. 



ORIGIN OF MINUTE LAMINATIONS OF CHERTS 



In both the Franciscan and Monterey groups, the beds of chert 

 are found to be laminated. As noted, this is quite common in the 

 Monterey cherts, and less frequent in the Franciscan. The suggestion 

 has been made that these laminations mark yearly alternations of con- 

 dition, while the major bedding planes may be due to cyclic changes 

 or due to some other cause than climatic variations. In the study of 

 Franciscan cherts, no evidence was obtained on this point, but in the 

 case of the Monterey cherts it is quite clear that this lamination is 

 not due to seasonal variations. This is shown by the fact that many 

 of the lines of lamination close upon themselves in elliptical curves, 

 suggesting concretionary action of some sort. 



Objections to Both Hypotheses of Climatic Variation 



In both the preceding hypotheses for explaining the rhythmic bed- 

 ding by climatic changes, a rather simple method of transportation of 

 sediment has been assumed. It has been assumed that the currents 

 from rivers, entering the ocean, came into still water and that the 

 final resting place of the sediment was determined by the strength of 

 the river currents and the distance to which they were able to main- 

 tain themselves through the more quiet water of the ocean. In this 

 way alternations in the volume of rivers would be competent to pro- 

 duce alternations in the nature of the sediments at certain special 

 areas in the basin of deposition. 



It would appear from the distribution of terrigenous muds that 

 the conditions were much more complex than this. If a map showing 

 the distribution of areas of terrigenous sediments be examined, it will 

 be found that the fine terrigenous muds form rather uniform bands 

 along the continental margins. They show no unusual seaward exten- 



159 Weather Cycles in the Growth of Big Trees, Mon. Weather Rev., pp. 225- 

 237, June, 1909. 



