abstracts: geology 223 



the different metals in the original ores have been concentrated in zones. 

 Thus, in the Horn Silver mine the surface zone is rich in lead and sil- 

 ver; beneath this is a zone rich in copper and zinc; and at still greater 

 depth is the original ore in which these metals are all present. This 

 rearrangement of the metal content has been an important factor in 

 determining the commercial value of the ore deposits. B. S. B. 



GEOLOGY. — The Mount Lyell copper district of Tasmania. Chester 

 G. Gilbert and Joseph E. Pogue. Proceedings of the U. S. 

 National Museum 45: 609-625, pis. 48-51. July 22, 1913. 

 This paper is based upon microscopic and metallographic study of a 

 comprehensive collection of rocks and ores in the. National Museum. 

 The ore deposits include (1) a pyritic mass of remarkable size lying 

 wdthin deeply dipping sericite schists adjacent to a faulted conglomerate 

 contact and consisting of fine-grained, homogeneous pyrite bearing dis- 

 seminated chalcopyrite and localized areas of enargite and tetrahedrite; 

 and (2) lenticular masses of bornite carrying subordinate chalcocite and 

 some tetrahedrite, pyrite, and chalcopyrite, in sericitic and chloritic 

 schists. Microscopic study points unmistakably to the formation of the 

 ores through replacement of the minerals of the schists. The study leads 

 also to the conclusion that the ore deposition took place during a dis- 

 tinct mineralizing epoch marked by solutions progressively changing in 

 composition and depositing a series of sulphide minerals in sequential 

 and transitional stages. The order of deposition runs from pyrite 

 through chalcopyrite, bornite, and chalcocite, to the tetrahedrite-enar- 

 gite group (accompanied by chalcopyrite of a second generation) . This 

 succession is in exact harmony with the order of increasing copper con- 

 tent and with that of decreasing iron content. It is believed that the 

 ore-bearing solutions were a deep-seated development from a differ- 

 entiating mass of igneous rocks, and that these solutions rose along struc- 

 turally developed channels, changing gradually in composition from the 

 beginning to the end of the depositional epoch. 



The paper gives a general account of the history and geology of the 

 district; a detailed description of the ore minerals, accompanied by two 

 photographs and eleven microphotographs of polished ore sections, two 

 showing a crystallographic intergrowth of bornite and chalcocite; a 

 brief discussion of the important analogous deposits of the world; and 

 closes with a bibliography. J. E. P. 



