REVIEWS 295 



merely been traced into Nebraska recently and incidentally rather than 

 as a part of a critical revisionary study, and (4) that the original locality 

 remains quite as accessible and more typical than the localities in 

 Nebraska, unite to make this proposal of a fourth title a proceeding of 



doubtful wisdom. 



T. C. C. 



Die Diamantfuhrenden Gesteine Siidafrikas. Ihr Ahhau und ihre 

 Aufhereitung. By Dr. Percy A. Wagner. Pp. 207, 29 text 

 figures, and 2 plates. Berlin: Gebriider Borntraeger, 1909. 



One of the fascinating problems of South African geology is the forma- 

 tion of the peculiar kimberlite pipes and the origin of the diamonds which 

 have made them so famous. This new work will be welcomed as a con- 

 tribution to that interesting subject. It describes carefully the different 

 phases of the kimberlite which fills the pipes, from the but scarcely altered 

 hardibank through the hard blue ground, soft blue ground, to the com- 

 pletely altered, hydrated, and well-oxidized yellow ground near the 

 surface. The point is made that within the pipes distinct zones can be 

 traced from top to bottom. Because certain individual characteristics, both 

 of the general material and of the diamonds as well, can be traced through- 

 out each zone, several observers have concluded that the pipes were formed 

 and filled by several distinct volcanic eruptions. But the nature of the erup- 

 tions which produced these great pipes, and at the same time filled them 

 with breccia, volcanic tuff, and a rather heterogeneous mass of material 

 derived in part from the deep-seated rocks, but containing also fragments of 

 sedimentary formations occurring at higher horizons, has not been brought 

 out very clearly and is perhaps not yet well understood. Contact meta- 

 morphic phenomena appear to be of rather limited extent. 



An interesting feature described by the author is that, on account of the 

 serpentinization of the olivine in the filling of the pipes, the material has, 

 in some cases, undergone a marked increase in volume. This increase in 

 volume has caused the filling to swell upward and this process has resulted 

 in the striation of the walls of the pipe. 



The three principal hypotheses of the origin of the diamonds are dis- 

 cussed. The author argues against the view held by some South African 

 geologists that the diamonds have been derived from disrupted deep- 

 seated eclogites, and favors the theory that the diamonds were developed 

 in the midst of the magma from its own inherent constituents. 



Chapters describing in detail the minerals of the pipes and dikes and 

 dealing with the petrography make up about half of the book. The work 



