112 T. H. Withers — Some Palaeozoic Fossils 



13. Conclusions of Part II. 



1. The effect of the generation of radio-thermal energy in the earth's 

 crust is to slow down the normal rate of cooling, and it is shown 

 that if the earth had originally a temperature of 1000° C, at, or just 

 below the surface, the effect of radio-activity sufficient to maintain 

 nearly three-fourths of the present flow of heat would be to increase 

 the period of cooling from twenty-two million years to 1,600 million 

 years. 



2. The method of estimating geological time by lead-uranium 

 ratios in radio-active minerals indicates that the oldest igneous rocks 

 of the earth's crust have an age of about 1,500 million years. 



3. Geological and other evidence points favourably to the 

 traditional view that the earth's crust was initially in a molten state. 



4. If the earth has cooled down from such a state to its present 

 thermal condition, nearly three-fourths of the present heat output 

 must be supplied by radio-activity. 



5. Volcanic temperatures can be more satisfactorily obtained in 

 depth on this view (4) than if the whole of the present temperature 

 gradient is maintained by radio-activity (Part I). 



6. There is nothing in the distribution of the radio-active elements 

 either superficially or in depth to forbid belief in an earth which 

 began with a molten surface, and which has gradually cooled down 

 to its present condition. 



Finally, I wish to express my thanks to Dr. Evans for valuable 

 suggestions and salutary criticisms offered during many discussions 

 on the subject-matter of this paper. 



III. — Some Paleozoic Fossils referred to the Cirripedia. 

 By Thomas H. Withers, F.G.S. 



SCATTERED throughout the Palseozoic rocks there are frequently 

 found certain fossils, in most cases represented by single detached 

 plates, which have been ascribed by various authors to the sub-class 

 Cirripedia. Thus we have fossils described under the names Balanas, 

 Cirripodites, Lepidocoleus, JPlumulites, Pollicipes, Protobalanus, Scal- 

 pellum, Stenotheca, Strobilepas, and Turrilepas, but in the case of 

 Balanus, Pollicipes, and Scalpellum, the reference to those recent 

 genera is quite unsupported by the structure of the fossils. 



While the Cirripede nature of some of these fossils appears to be 

 undoubted, others as certainly are not Cirripedes, and for the 

 remainder it would seem that, except for the similarity in ornament 

 to the valves of Cirripedes, the main reason for such reference is the 

 great difficulty in referring them to any other class of animals. 



It is only occasionally that the plates of these forms are found in 

 position so that we can gain some idea of what the complete shell was 

 like, and although more or less complete specimens have been found 

 of most of them, in no case is it possible to learn the precise relation- 

 ship of the animal to the shell. 



These brief notes are published in order to clear the ground for 

 a more detailed study of some of the above-mentioned forms. I wish 



