322 Revietvs — Lydekker^s Contributions to 



and the biotite are decomposed into chlorite, and that a break occurs 

 somewhere in this intermediate chloritized rock. But this view 

 labours under enormous difficulties. For : — 



(1) There is no evidence of a break, either in the field or the 

 microscope. The changes that take place, whether structural or 

 mineral, proceed gradatim, and are similar in numberless sections. 



(2) The mineral transformations proceed pari passu with the 

 mechanical deformation ; hornblende, for example, in the ordinary 

 diorite changing to chlorite in the crushed and slightly sheared rock, 

 the chlorite, in its turn, passing into biotite in the core of the shear- 

 zones. The felspar also undergoes transformations corresponding to 

 the degree of shearing. 



(3) The uniform appearance of mica where the shearing is great 

 and where the granite-veins are numerous, while it is nowhere seen 

 where shearing and veins are absent, appears almost to demonstrate 

 that these are the true causes of the generation of the mica. It 

 would be strange, indeed, if such powerful causes produced no 

 effect. That the mineral changes gradually increase as we approach 

 the veined complex is quite inconsistent with the hypothesis of a 

 break. If there be no break, it appears to me that my conclusion is 

 demonstrated. 



My excuse for again troubling the readers of this Magazine on 

 this subject is that it has a significance beyond the mere mineral 

 change. The origin of the biotite virtually carries with it the whole 

 theory of the metamorphism at Malvern. 



la iE V I E -VT" S. 



I. — Pal^ontologia Argentina. II. Contributions to a Know- 

 ledge OF THE Fossil Vertebrata of Argentina. By R. 

 LvDEKKEK. (Annales del Museo de la Plata.) La Plata, 1893. 

 (Edited by Dr. F. P. Moreno, Director of the Museum.) 



THIS beautifully printed and illustrated volume is the outcome 

 of work done by Mr. Lydekker during a short visit to the 

 Argentine Eepublic in the autumn of last year. The confusion that 

 had arisen in the nomenclature of the extinct mammalian fauna of 

 South America was very great, and it was desirable that someone 

 well acquainted with the vertebrate paleontology of the Old World 

 should have an opportunity of examining the material which had 

 been the subject of so many papers, in some of which, it was 

 evident, new genera and species had been founded with a plentiful 

 lack of discretion. The announcement, therefore, that Mr. Lydekker 

 was about to visit the La Plata Museum, where many of the types 

 are preserved, was very welcome, and, in spite of the shortness of 

 the time at his disposal, he has been able to make important additions 

 to our knowledge of the fossil vertebrates of South America. The 

 present volume contains three Memoirs : 



(1) The Dinosaurs of Patagonia. 



(2) Cetacean Skulls from Patagonia. 



(3) A Study of Extinct Argentine Ungulates. 



