8 THE TIDAL PROBLEM. 



Working upon the cosmogonic grounds prevalent in the past century, 

 and supported by the nearly universal consensus of opinion regarding the 

 early stages of the earth, Sir George Darwin, in a memorable series of 

 mathematical investigations/ developed the well-known doctrine of the 

 tidal retardation of the earth's rotation from a primitive period of less 

 than 5 hours 36 minutes to the present period of four times that length. 

 Besides being grounded in presumptions that were commonly accepted, 

 it had the merit of bringing these presumptions into historical consistency 

 with the existing state of things. Not only that, but the investigation 

 started with what then seemed to be a present rate of retardation deduced 

 from astronomical observations, and proceeded backward by logical steps 

 and current assumptions to the supposed original state, or at least to a 

 close approach to it. The confidence that has been reposed in the conclu- 

 sions so reached has not been placed without persuasive reasons, whatever 

 conclusions may ultimately be reached from radically different cosmogonic 

 postulates and from revised astronomical data. 



Not a few inferences of vital geological importance were drawn from 

 this classic investigation, and specific data to support them were naturally 

 sought in the geologic record. For the greater part, this search met with 

 negative results, or with results which could be regarded as giving but 

 meager or equivocal confirmation. Notwithstanding this, the logical force 

 of the tidal argument as developed by Sir George Darwin, when its cos- 

 mogonic postulates were taken for granted, was such that inharmonious 

 geological phenomena were generally explained away, largely by assuming 

 that the internal solidification of the earth took place at a relatively late 

 date. 



CONSIDERATIONS BASED ON THE PLANETESIMAL THEORY. 



As to the grounds for postulating a radically different constitution of 

 the lithosphere, growing out of a new hypothesis of earth-genesis, I must 

 content myself here with references to what has already been written ^ 

 and to a fuller exposition elsewhere in this series of papers. It is appro- 

 priate, however, to bring again to mind those inferences which are drawn 

 from the rotational features now shown by the solar system, since these 

 bear specifically upon the question in hand. 



The doctrine that a prevalent forward rotation of the planets could 

 only mean that they were formed through gaseous or quasi-gaseous con- 

 densation, was one of the bulwarks of the older hypotheses. It was only 



2 A OTOup of hypotheses bearing on climatic changes, T. C. ChamberHn, Jour. Geol., 



vol. 5, No. 7, 1897, pp. 653-683. 

 An attempt to test the nebular hypothesis by the relations of masses and momenta, 



T. C. Chamberlin, Jour. Geol., vol. 8, No. 1, Jan.-Feb., 1900, pp. 58-73. 

 An attempt to test the nebular hypothesis by an appeal to the laws of dynamics, 



F. R. Moulton, Astrophys. Jour., vol. 6, No. 2, Mar., 1900, pp. 103-130. 

 Certain recent attempts to test the nebular hypothesis, T. C. Chamberlin and F. R. 



Moulton, Science, vol. 12, Aug. 10, 1900. , ^ , , oi 



The origin of the earth, Chamberhn and Salisbury, chap. I, vol. 2, Geology, pp. 1-»1 



Dec, 1905. 

 Evolution of the solar system, F. R. Moulton, chap. XV, Introduction to Astronomy, 



pp. 440-487, Mar. 24, 1906. 



