Reviews — A Textbook of Mineralogy. 329 



M. Martel attempts to lay down definitions that will help to 

 introduce uniformity in the common usage of several terms which, 

 in referring especially to those associated with porosity, he 

 cliaracterizes as a veritable confusion of tongues descended straight 

 from the tower of Babel. He thinks that one ought to regard as 

 'porous those rocks which are penetrable by water but which, unlike 

 those that are jiermeaMe, are not entirely traversed by it ; they 

 retain in their interstices (or pores) a proportion more or less great 

 of the water which succeeds in percolating only under pressure. 

 Porosity is thus a state intermediate between permeability and 

 impermeability, in consequence of a conflict between capillarity 

 and gravity. M. Martel classifies rocks according to their behaviour 

 to water, as (1) impermeable, (2) porous and impermeable, (3) porous 

 and permeable, (4) permeable without porosity. This restriction 

 of the term porosity is at variance with the common usage of oil 

 geologists, but, without discussing at this stage the merits of 

 alternative terms, everyone will agree that an advantageous 

 uniformity will be gained by adopting M. Martel's definitions. 



Similarly, the author attempts to correct popular usage by 

 defining the terms attached to springs. He uses as a general term 

 emergence to cover (a) a spring of naturally filtered water which issues 

 from permeable detrital rocks, and (6) a resurgence, or the 

 reappearance of an underground river, such as one meets in a lime- 

 stone country. The term fountain is lirrited to an artificially 

 developed emergence. 



Although the subjects are well classified, with numerous sub- 

 divisions distinctly indicated by expressive headmgs, the book 

 loses seriously in value as a work of reference by the absence of 

 alphabetical indices of authors, localities, technical terms (especially 

 those of local application) and subjects. This shortcoming could be 

 repaired by an English translation, which would serve to arouse 

 interest in a subject that has received far too little attention by 

 English naturalists, chemists, medical men, and engineers. 



T. H. Holland. 



A Textbook of Mineralogy. By E. S. Dana. Third edition, 



revised and enlarged, by W. E. Ford. pp. ix + 720, with 



1,050 figures. New York : Wiley. London : Chapman and 



Hall, 1922. Price 25s. net. 



rPHE present writer learnt his descriptive mineralogy almost 



-^ entirely from the " little Dana ", with occasional excursions 



in the direction of the " big Dana " for minute detail in special cases. 



Being a person of conservative tendencies he has continued to use 



Dana, in spite of the great number of admirable modern Ameiican 



and German books of reference now on the market. "With much 



pleasure, therefore, he welcomes the appearance of a third and 



up-to-date edition of the Textbook, combining all the old familiar 



features with modernization of those parts of the subject that 



