Reviews — Silver Ores. 47 



possible importance, for while extensions are naturally to be sought 

 in existing fields, it is just as vital that attention be focussed on the 

 location of new and likely areas whose resources are as yet imperfectly 

 realized. The small oil map of the world included on page 89 shows 

 in a striking manner, however, the unfortunate divorce between 

 the geographical situations of our colonies and those of the main 

 petroliferous provinces of the world ; so much country has 

 unfortunately to be " ruled out " as impossible, from tectonic 

 considerations. 



Of the foreign resources one of the most interesting areas is 

 undoubtedly that of Sakhalin (Russia) with its oil belt stretching 

 along the Pacific coast for nearly 200 miles ; though geologically 

 complex, the indications are good, and an asphalt lake near Oha 

 Creek, alleged to contain 25,000 tons of raw material, is not the least 

 of these. Parts of Central America and Colombia also hold out 

 great possibilities, while exploration in Venezuela, though slow and 

 intermittent in the past, is now meeting with deserved success. 



Statistics of production are given in most cases, together with the 

 results of refining the various crude oils obtained. The amount of 

 geographical and geological information included is surprising for 

 a volume of this size ; it is essentially the embodiment of this 

 information to the exclusion of irrelevant commercial matter that 

 sets a real value on the monograph, and we may express the hope 

 that it will be widely read, not only by the scientific worker, but also 

 by the general public. 



H. B. MiLNER. 



Silver Ores. By H. B. Cronshaw. Imperial Institute Mono- 

 graphs, pp. ix: + 152, with 1 map. London : Murray, 1921. 

 Price 65. net. 



/^NE of the most striking facts concerning the silver production 

 ^ of the world from the mining and geological point of view is 

 that about four-fifths of it is obtained as a by-product from the 

 mining and metallurgical treatment of ores exploited mainly or 

 largely for other metals, gold, copper, lead, and zinc. In many cases, 

 however, the economic working of these ores would be impossible 

 but for the added value derived from the silver. Hence it is some- 

 what difficult to define what is really an ore of silver. The ore- 

 deposits in which silver is the dominant metal show many interesting 

 features, especially with regard to the phenomena of oxidation and 

 formation of zones of secondary enrichment, while some of the 

 deposits of native silver are very remarkable. 



In this monograph are brought together accounts of all the 

 important silver deposits of the world ; in a few cases, where the 

 silver is essentially a by-product, the author has wisely contented 

 himself with giving references to other works, when such are well- 

 known or easily accessible ; such are, for example, the lead-silver 

 ores of England. It is a curious fact that the galena of Yorkshire 



