July 7, 192 1] 



NATURE 



5«i 



tion of the type locality, the general distribution, 

 references to illustrations, and general notes are 

 given. All the descriptions are in English, and 

 written in a correct but very simplified style, so 

 that anyone can easily understand them. Latin 

 descriptions find no place in this admirable work, 

 which is designed to be useful to the multitude 

 rather than to the botanist alone. 



In the first volume the Cactaceae are divided 

 into the three tribes Pereskieae, Opuntieae, and 

 Cereae. The tribe Pereskieae contains only the 

 single genus Pereskia, of which nineteen species 

 are described. The Opuntieae are divided into seven 

 genera, of which Opuntia is the largest, contain- 

 ing 240 species ; the other genera have only one or 

 a few species in each. These two tribes fill the 

 first volume, and the Cereae, which comprise the 

 bulk of the order, are being dealt with in the 

 remaining volumes. The first portion of the Cereae 

 is accounted for in the present volume, where the 

 plants that are commonly known by the generic 

 title of Cereus are described and illustrated. 



It has long been recognised that different species 

 of Cereus produced different types of flower, and 

 since, in other natural orders, differences in floral 

 structure are recognised as being of generic value, 

 some botanists have founded genera upon some 

 of the different floral types found among Cerei. 

 These genera have not hitherto found much 

 favour among botanists or horticulturists, because 

 the plants, when not in flower, often present a 

 great similarity to one another. The authors of 

 "The Cactaceae," however, have accepted the view 

 that a difference in floral characters should con- 

 stitute a generic distinction; they have had this 

 view constantly in mind, and carried it to a 

 logical conclusion, so that they have divided the 

 old genus Cereus into no fewer than forty-seven 

 genera, containing 275 species. Twenty of the 

 genera are proposed for the first time in this 

 volume. Whilst there can be little doubt that the 

 plants which have been placed in the genera 

 Heliocereus, Aporocactus, Cleistocactus, etc., 

 should be separated from Cereus, it may be ques- 

 tioned whether those placed in the genera Den- 

 drocereus, Harrisia, Acanthocereus, etc., should 

 be held to constitute more than sectional groups 

 under Cereus. This, however, is a matter of 

 opinion, and time alone will show if the numerous 

 genera maintained in this work will be generally 

 accepted. Whether they are accepted or not, 

 their adoption in " The Cactaceae " in no way in- 

 validates the usefulness of that splendid work. 

 The reviewer has had considerable experience in 

 the use of existing monographs, and has found 

 " The Cactaceae " very far in advance of them ; 

 NO. 2697, VOL. 107] 



it can be recommended with confidence to 

 students. 



The only noticeable fault in the work is the 

 rather serious one that exceptions are always 

 neglected in the keys. This, however, is a fault 

 appertaining to most botanical works, and invari'- 

 ably leads the novice astray. For example, should 

 a novice desire to find out, by means of the key 

 given on p. i of vol. ii., the subtribe in which 

 the authors place the globose, spineless plant 

 cultivated under the name Echino cactus (and 

 Anhalonium) Williatnsii, no place for it will be 

 found. The only spineless plants mentioned in 

 that key belong to the subtribe Epiphyllanae, 

 which have flat, many-jointed stems. This fault 

 is so easily remedied by including exceptions and 

 variable characters again and again under the 

 headings of different parts of the key that per- 

 haps the authors will endeavour to correct this 

 defect in the continuation of their work. They 

 are to be congratulated heartily upon the 

 manner in which they have so far carried out their 

 very difficult task of evolving order out of the very 

 confused synonymy found in other works upon 

 this group of plants. N. E. Brown. 



A Study in Geo-chemistry. 

 The Enrichment of Ore Deposits. By W. H. 

 Emmons. (Bulletin 625. United States Geo- 

 logical Survey : Department of the Interior.) 

 Pp- 5SO- (Washington : Government Printing 

 Office, 1917.) 



UNDER the above modest guise the United 

 States Geological Survey has published a 

 volume of the greatest value to the student of ore 

 deposition, which may fairly claim to rank as one 

 of the most interesting of recent contributions to 

 this very difficult branch of economic geology. The 

 author points out that two phases in the formation 

 of economically important ore deposits require inde- 

 pendent investigation, namely, the formation o^ 

 the primary ore deposits and the subsequent modi- 

 fications which the more superficial portions in 

 many cases undergo. 



The present work is devoted exclusively to a 

 comprehensive investigation of the second of these 

 phases, the genesis of the primary ore deposits 

 being considered indirectly only. The author 

 reviews successively the conditions that make for 

 enrichment, such as amount of rainfall, surface 

 contours, permeability of the rocks, the nature of 

 the underground circulation of water, and in par- 

 ticular the oxidation of sulphide deposits. He lays 

 very great stress upon the last-named, and 

 ascribes the solution of the various nnlneral^ 



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