566 Revieics — Thomm ^ MacAlkter — Ove Beposiis. 



sympathy goes out to the authors when they say that they have 

 found it fraught with much difficulty. But the work has been placed 

 in good hands, and although others perhaps, under the same circum- 

 stances, might have sacrificed the numerous short notices of illustrative 

 examples for more detailed descriptions of a few occurrences of ore, 

 and abundant references to original works, the volume under review 

 contains a wealth of information ; and if, on closing it, we feel 

 inclined to "ask for moi'e ", we may be sure that had the authors 

 had greater space at their disposal, full justice would have been done 

 to the subject. 



The first chapter is an Introduction of twenty pages, that contain 

 matter capable of expansion to four times the space. Then follow 

 eight chapters, in which the authors classify ore deposits according to 

 their mode of origin, under the following headings : — 



Ores due to the diilerentiation of igneous magmas. 



Pneumatolysis. 



Hydatognesis — deposits formed by after-eruptive actions which are 

 not pneumatolytic. 



Ores due to metasomatic replacement. 



Eedded ores due to precipitation. 



Metamorphic ore deposits. 



Secondary changes in ore deposits apart from metamorphism. 



Detrital and alluvial deposits. 



As the authors point out, any classification of ore deposits can only 

 be arbitrary, and they themselves mention one objection to the above 

 that will instantly occur to the reader, the difficulty of separating 

 deposits formed by pneumatolysis from those formed by metasomatism, 

 which is surmounted in this case by restricting metasomatism to 

 aqueous solutions only. IS^evertheless, this classification is easily 

 followed and therefore good. 



The best chapters in the book are, we think, that on ores due 

 to the differentiation of igneous magmas and that on secondary 

 changes. The illustrations are fair ; but the index, which gives no 

 names of localities, leaves much to be desired. 



It would be unkind to the authors of this work to dwell on certain 

 defects in the matter of clearness of statement, such, for instance, as 

 that on p. 301, where precipitation is asserted to be dependent in all 

 cases on the liquid being in a state of supersaturation with regard to 

 the substance undergoing precipitation ; or that on p. 14, where we 

 are told that practically all tin-lodes are characterized by ' greiseniza- 

 tion ' of their walls. The former is logically correct, but requires 

 explanation, Avhile the second demands a very wide interpretation for 

 the term ' greiseu ' before it can be accepted. However, such defects 

 are perhaps difficult to avoid where 'boiling-down' is imperative, 

 although a slight want of care in some quotations cannot be similarly 

 excused. Again, detailed discussions on debatable points are, we 

 know, out of the question, but we doubt whether ever)^ reader will 

 agree with the authors in their treatment of the Witwatersrand 

 conglomerates. In such a work as this it is surely better to state 

 the arguments for and against, and leave the reader to form his own 

 conclusion. 



