Geology and Mineralogy. 249 



Geol. Survey, 1890. — The following important conclusions are de- 

 duced by Mr. Iddings from his study of the Tewan volcanic rocks. 

 (1) The collection of rocks from the Tewan mountains, New 

 Mexico, though small, shows a gradual transition in mineral com- 

 position from rhyolites through andesitic rocks to basalts. (2) 

 The whole series is characterized by a variable amount of porphy- 

 ritical quartz in rounded grains, which is very noticeable in some 

 of the basalts. (3) These quartzes are primary secretions or 

 crystallizations from the molten magma and exhibit no definite 

 relation to its chemical composition, being present, in or absent 

 from, rocks of similar chemical composition. (4) Their produc- 

 tion is to be referred to certain physical conditions attending 

 some earlier period of the magma's existence. Mr. Iddings also 

 infers (5) from analogy with the occurrence of iron-olivine in 

 rhyolitic obsidian, that the formation of the quartz in the basalt 

 took place through the influence of water-vapor while the magma 

 was under considerable pressure. 



9. Elementary Geology ; by Charles Bird. pp. i-vi, 1-248, 

 247 illustrations and map. Longman's Elementary Science Man- 

 uals, 1890. — A convenient compilation of the general facts of 

 Geology in attractive form and especially adapted to the British 

 student. It is designed primarily to arouse an interest in natural 

 phenomena and outdoor objects, and also to enable the student to 

 gain, through examination, a South Kensington certificate. In 

 subsequent editions, it would be well to orient a number of the 

 illustrations which now are placed in various unnatural positions. 



10. Allgemeine unci Chemische Geologie. — Dritter band. Erste 

 Abtheilung. Allgemeine Geologie von Justus Roth, 210 pp. 

 Berlin, 1890, (Wilhelm Hertz). The third volume is now begun 

 of this well known and highly valued work. The greater part of 

 the number is given to the discussion of the subject of metamor- 

 phism, more especially that of local or contact nature. So many 

 important facts and observations have been gradually collected 

 on this last subject, that their collection together and orderly 

 arrangement are a great advantage to all interested in this 

 department of geology. 



11. On Black Rittile from the Black Hills ; by W. P. Head- 

 dex, with a note on the crystals by L. V. Pirsson (communi- 

 cated). — The material which forms the subject of this note was 

 found among some specimens of tin ore in the cabinet of the 

 School of Mines of South Dakota. The specimen was without 

 label and no locality was attached to it; it is, however, quite cer- 

 tain that it is from the Harney Peak district of the Black Hills, 

 probably from the immediate neighborhood of the Etta mine, 

 Pennington Co. The crystals formed aggregates usually sur- 

 rounded by and always associated with muscovite in a mass of 

 feldspar. The crystals are small, deep black and have a brilliant 

 metallic luster; in form they have the aspect of orthorhombic 

 prisms terminated by a macrodome. Hardness, 6 ; Sp. gravity, 

 5*294. Streak and powder, grayish-black. BB. unchanged and 



