GEOLOGY. 307 



GEOLOGY. 



Chamberlin, T. C, University of Chicago, Chicago, lUinois. Study of funda- 

 mental 'prohlems of geology. (For previous reports see Year Books Nos. 2-15.) 



In my last report of progress (Year Book No. 15, pp. 358-359) 

 a purpose was expressed to limit further cosmogonic studies, at least 

 for the present, to a report on such collateral work as had been under- 

 taken incidentally in the search for the genesis of the earth — on which 

 reports have alreadj^ been made — and to devote future studies to prob- 

 lems closely related to the earth itself, particularly questions relating 

 to the major dynamics of earth-shaping and to the climatic states of the 

 geologic ages. This purpose has controlled the work of the year in the 

 main, but not entirely. A new phase of cosmogonic inquiry has grown 

 out of the recent success of observers in the measurement of the veloc- 

 ities of nebulae, and this has seemed to make a further study of 

 nebular theories advisable. 



While the velocities of only a small fraction of known nebulae are as 

 yet determined — and these determinations are to be held in some meas- 

 ure tentative — the results leave little room for doubt that the mean 

 orbital speeds of nebula) are notably greater than those of stars. This 

 does not apply equally to all classes of nebulae, but has significance 

 in two of the four leading classes : (1) Certain nebulae, and certain vague 

 nebulosities scarcely well enough defined to be called nebulae, seem to be 

 so physically connected with certain stars or star-clusters that they must 

 be regarded as having essentially common velocities. (2) Certain 

 nebulae whose forms suggest an origin by collision have low velocities. 

 (3) Planetary nebulae that seem to be free from bondage to stars or star- 

 groups have velocities whose mean is about seven times as great as that 

 of the blue starsUo which they are nearest in spectroscopic character and 

 presumably in evolution . (4) Spiral nebulae, so far as determined, disclose 

 a mean velocity twenty or thirty times as great as the mean velocity of 

 white stars to which they seem spectroscopically most nearly related. 



Probably a closer discrimination of classes will be required when 

 velocities are more generally determined, but the data already acquired 

 call for inquiry into possible sources of enhanced nebular velocity and 

 the significance of such velocity in evolution. 



The Revived Herschellian Hypothesis. 



Incidentally, the disclosure of the high velocities of spiral nebulae 

 has stimulated a reconsideration of the view of the elder Herschel 

 that nebulae are merely dim, distant star-clusters of the order of our 

 galactic system. Of course, the bright-line nebulae are omitted from 



*W. W. Campbell, presidential address before the American Association for the Advancement 

 of Science, delivered in the American Museum of Natural Historj', New York City, Dec. 26, 1916. 



