60 Arthur Holmes — Radio-activity 



EXPLANATION OF PLATE III. 



Pyrgocystis grayae. 



PlG. 1. The holotype seen in three-quarter view from above, so as to show 

 the oral face. 

 ,, 2. The holotype seen from the side. 



Both these figures are photographs from a squeeze of the original. 

 X 3 diam. 



Pyrgocystis ansticei. 



3. Specimen a. Side view. Note the irregular shape of the plates, 

 due to fracture. 



4. Specimen c. Holotype, side view. Note the regular plates arranged 

 in columns. 



5. Specimen e. Side view. Note the slightly irregular columns, of 

 which there are only 7. 



6. Specimen I. Side view. Note the small number of columns at the 

 distal end, with new ones coming in above. 



7. Specimen g. View of distal end. Since the turret is much curved, 

 its side is also visible, though out of focus. There is no trace of 

 a lumen. 



8. Specimen c. Holotype, view of distal end. The turret is slightly 

 curved. There is a small distinct lumen. 



9. Specimen e. View of distal end. The position of the lumen is 

 filled with a solid mass of plates and secondary mineral matter. 



10. Specimen m. Side view. Note the irregularity of the columns near 

 the tapering distal end. 



11. Specimen a. View of adoral end. Note the approach to horizontality 

 of the plates, and how they stretch across the lumen. 



12. Specimen c. Holotype, view of adoral end, as in Fig. 11. 



13. Specimen i. View of adoral end. Traces of the lumen are seen 



filled with matrix. 



All the preceding figures, 3-13, are from photographs, taken 



and worked up by the author. X 3 diam., the same scale as 



Aurivillius' figures. 

 ,, 14. Specimen c. Holotype. A plate at the distal end. x 7-5 diam. 

 ,, 15. Specimen,;'. A plate at the distal end. X 7-5 diam. 



Unfortunately it has not proved possible to obtain for comparison similar 

 drawings of the plates in the Swedish species. 



II. — Kadio-activttz and the Earth's Thermal Histoky. 



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



PAKT I. 



The Concentration of the Badio-active Elements in the Earth's Crust. 



1. Introduction. 



TWO years ago, in discussing the thermal energy of the earth, 1 

 I suggested that while it had become impossible to deduce the 

 earth's age from its thermal condition alone, Kelvin's problem might 

 profitably be reversed by accepting the earth's age as a known factor, 

 and deducing with its help the thermal history of the earth. This 

 paper is a first attempt to attack the new problem then suggested. 

 For geological purposes, one of the most fundamental aspects of the 



1 The Age of the Earth, p. 135, 1913. 



