REVIEWS 95 



a bearing on geological processes. To a large extent the author has 

 drawn on his own work, and has provided material which it would be 

 very difficult, if not impossible, for the geologist to obtain elsewhere. 

 Most important are the experiments on "rhythmic precipitation" or 

 banded structure, which is formed, for instance, when silver nitrate 

 diffuses through a gel containing potassium bichromate. The silver 

 chromate forms in concentric bands separated by clear interspaces, at 

 once suggestive of the ring structure of the agate. 



In applying the results of the experiments to geological structure, 

 the author displays — for a specialist in another science — a remarkably 

 good knowledge of geological literature. Few economic geologists 

 would be prepared to give such an important place to lateral secretion 

 as the agency by which veins are filled, but many objections to the 

 theory are met by reference to actual experiment. The chief value of 

 the book lies in the application of "rhythmic precipitation" to agate 

 structure, banded flints, eozo5n, banded clay concretions, and "weather- 

 ing rings." The treatment of alternating gold and pyrite layers in 

 quartz from the same point of view will be subjected to question. 

 The author has at least added another to the explanations which have 

 been offered for the gold deposits of the Rand. 



Without doubt the author has provided geologists with material 

 which will be the means of opening up a new and fascinating field for 

 investigation in the very near future. 



R. C. Wallace 



Fosseis Devonianos do Parana. By Dr. John M. Clarke. "Mono- 

 graphias do Service Geologico e Mineralogico do Brazil," 

 Vol. I. Rio de Janeiro, 191 3. 



In his recent monograph on the Devonian faunas of the southern 

 hemisphere Dr. Clarke has placed Brazil in the same position of honor 

 in the southern continent that New York has always held in North 

 America, as the holder of the standard Devonian column and geologic 

 record of that time. The work in its philosophic treatment and broad 

 learning, as well as in its perfection of illustration and text, is beyond 

 all criticism. 



The paleogeography of thg Devonian is discussed thoroughly, and 

 a map is given showing the distribution of land and water in that age, 

 to express the relations of the faunas. This map shows the austral 

 continent connecting South America with South Africa, some central 



