Reviews — >S^. 8. Buckman — Yorkshire Type Ammonites. 377 



and, as all the corrections are based on insufficient data, it is shown 

 that the method is extremely unreliable. 



In the chapter on the relation of sedimentation to geological time 

 the previous estimates based on the thickness of sediments are 

 tabulated. Of these the three most recent, those of Greikie, Joly, 

 and Sollas, vary between 80 and 100 million years. The author 

 points out four further methods by which one arrives at ages varying 

 from 250 to 350 million years, with a possible upper limit of 

 700 million. It is held that we may be living in an age of more 

 than average sedimentation, in support of which opinion a communica- 

 tion from Professor Chamberlain is quoted. 



The author has no confidence in purely geological methods for 

 determining the earth's age. He is, however, enthusiastic on the 

 possibilities brought to light by the study of radio-activity in the 

 rocks, a subject on which he has done much original work. 



The accounts of recent research on radio-activity, of the disintegra- 

 tion theory, and of the measurement of radio-activity in rocks, are 

 well written. The work of Joly and Fletcher on pleochroic haloes 

 is reviewed, and it is pointed out that these haloes, after further 

 study, may be used as an indication of the ages of the minerals in 

 which they occur. 



The chapter on radio-active minerals is a very important one, 

 giving a lucid account of the refined methods of estimating the age 

 of rocks by determining the ratio of helium to uranium, or of lead 

 to uranium, in the radio-active uranium minerals. The work of 

 Strutt, Boltwood, and Rutherfo'rd is described, and the author has 

 also given us the results of some of his own experiments. Por 

 Archaean rocks measurements of the ' ' helium ratio ' give an age 

 of 700 million years ; measurements of the ' lead ratio ', however, 

 give double that figure. 



In the last chapter the author reviews all the evidence, and 

 endeavours to show in what directions one must look for a recon- 

 ciliation between the new and the older methods of determining the 

 age of the earth. 



There are two useful appendices, one giving various data referi'ed 

 to in the chapters on radio-activity, the other giving a very complete 

 bibliography of the whole scope of the book. 



In the small space at his disposal the author has handled a vast 

 subject in a very able manner. 



YI. — Yorkshire Type Ammonites. Edited by S. S. Buckman. The 

 original descriptions reprinted and illustrated by figures of the types 

 reproduced from photographs mainly by J. W. Tutcher. Part ix, 

 10 plates, with descriptions. London : Wesley, 1913. 



WITH this part begins vol. ii of this exceedingly important work. 

 It was long recognized that until some master hand had 

 examined, redescribed, and figured the numerous species of Lias 

 Ammonites from Yorkshire that had been named by Martin Simpson 

 little progress could be made in this fauna. Mr. Buckman essayed 

 the task, and every species that he deals with clears up ground of 

 a most uncertain and treacherous nature. And now that he is well 



