102 Arthur Holmes — Radio-activity 



The first is clearly of the type of Schhtheimia Greenoughi, though 

 just a trifle too evolute, since its umbilicus measures 24 per cent, 

 instead of 21 per cent at the same diameter. It is also too densely 

 costate, as is d'Orbigny's original figure of A. Charmassei 1 (especially 

 on the inner whorls) and Wright's ' Mgoceras'' Charmassei. 2 The 

 latter also has a silicate periphery to a larger diameter than the 

 others, and Dumortier's A. Charmassei 3 is distinguished by its 

 flexicostation, as well as by the closeness of its ornament. 



S. intermedia, Pompeckj, greatly resembles the two large forms 

 that are here considered to be allied to S. Greenoughi,. but may be 

 a little thinner, though the exact thickness of the latter cannot 

 be determined owing to the corrosion of one side of the specimens. 

 The costation differs in being a little too close. 



S. oV Orbignyana (Hyatt), Pompeckj, has at a diameter of 420 mm. 

 the dimensions of the considerably smaller example of S. Greenoughi, 

 i.e. it remains involute; but its costation also is too close and the 

 periphery too sharp. 



Finally, as an illustration of the excentric coiling of some forms 

 of Schhtheimia, it may be mentioned that in S. depressa (Warmer), 

 Pompeckj 4 ( = ^4. angulatus thalassieus, Quenstedt, 1883, pi. ii, fig. 4 

 only), the inclusion decreases from 41 to 28 per cent in less than 

 a whorl, whereas in S. thalassica, Quenstedt, sp. ( = A. angulatus 

 thalassieus, Quenstedt, 1883, pi. ii, fig. 5), the umbilicus, owing to 

 the rapid increase in height of the last whorl, decreases from 50 to 

 37 per cent at diameters of 32 and 100 mm. respectively. 



In conclusion, I should like to express my best thanks to Dr. A. 

 Smith Woodward, through whose kind offices I was permitted to 

 work on the rich store of Ammonites in the British Museum, and to 

 Mr. G. C. Crick for placing his extensive knowledge of the collection 

 at my disposal. 



II. — Radio-activity; and the Earth's Thermal History. 

 By Arthur Holmes, A.E.C.S., D.I.C., B.Sc, F.G.S. 



PABT II. 



Radio-activity and the Earth as a Cooling Body. 5 



7. Other Sources of Terrestrial Heat. 



SO far, the source of the heat lost from the earth has been assigned 

 only to the disintegration of the radio-active elements. Besides 

 this, heat is liberated near the surface by the processes of weathering. 

 The decomposition of one gram of average rock is accompanied by the 

 generation of about 120 calories. Owing to its extreme slowness, this 



1 A. d'Orbigny, " Paleont. Frang. Terr. Jurass. Ceph.," pi. xci, figs. 3, 4, 

 non 1-2, nee pi. xcii (see S. S. Buckman, " Some Lias Amm. " : Proc. Cottes- 

 wold Club, vol. xv, pt. hi, p. 239, 1906). 



2 T. Wright, op. cit., 1880, pi. xx. 



3 E. Dumortier, op. cit., 1867, p. 29, pi. xvii, fig. 1. 



4 See J. F. Pompeckj, op. cit., p. 78, and F. Wahner, " Beitr. Kenntn. 

 tieferen Zonen d. Unt. Lias i. d. No. Alpen," part hi: Beitr. Pal. Osterr.- 

 Ung., vol. iv, p. 164, 1886. 



5 Part I, " The Concentration of the Radio-active Elements in the Earth's 

 Crust," appeared in the February Number, pp. 60-71. 



