368 Reviews — Dana's System of Mineralogy. 



optical and microscopical study having been developed and brought 

 within the reach of all mineralogical observers. New means of 

 observation have not only increased our knowledge of the optical 

 constants of many species, but have developed new views in regard 

 to the molecular structure of crystals. In Chemical Mineralogy, 

 also, there has been rapid progress ; on the theoretical side, in the 

 way of explaining the composition of complex species and groups 

 of species ; again on the analytical side, and perhaps even more by 

 the development of the synthetic processes. The last-mentioned 

 methods, in the hands of skilful chemists, have resulted in the 

 reproduction in the laboratory of most of the prominent mineral 

 species, as the felsj^ars, quartz, the pyroxenes and chrysolites, 

 amphibole, corundum, etc. ; thus throwing much light upon the 

 composition of species and their formation in nature. 



Great care has been bestowed by the author upon the crystallo- 

 graphic portion of the subject ; and the attempt has been made to 

 trace back to the original observer the fundamental angles for each 

 species — then the axes have been recalculated from them, and finally 

 the important angles for all common forms have been calculated 

 from these axes. Where there has been no other independent means 

 of verification at hand, the angles have, in most cases, been calculated 

 a second time independently. In this way the author hopes that 

 a fair degree of accuracy has been attained. The Millerian indices 

 have been adopted throughout. 



The habits of the crystals, methods of twinning, and the physical 

 characters, especially those on the optical side, have been carefully 

 re-written and in general are given with much fullness, In the 

 list of analyses, all are given that are likely to prove useful for a 

 complete understanding of the composition of each species. This 

 means all reliable analyses in the case of the rare species or those 

 of complex composition. 



In the Introduction, after explaining the order observed in the 

 description of the species and the abbreviations used, the author 

 devotes 20 pages to Crystallography, giving as briefl}' as possible 

 an explanation of the six systems of crystallization. Each system 

 is carefully illustrated, and tables of the more important angles 

 are given. The next subdivision treats of Physical Mineralogy — 

 (1) Characters depending upon cohesion; as Cleavage, Fracture, 

 Tenacity, Hardness. Then (2) Specific Gravity or Density. (3) 

 Characters depending upon light — as Lustre, Colour, Diaphaneity, 

 or degi-ee of transparency ; special optical properties and anomalies. 

 Uniaxial and Biaxial Crystals are then explained. (4) Characters 

 relating to Heat ; and (5) to Magnetism and Electricity. These 

 latter characters are so far special, that they are treated very briefly 

 in this work, under the individual species ; references, however, are 

 ' given to the most important papers on the subject. 



The next section is Chemical Mineralogy : the classification 

 adopted in this work follows, first, the chemical composition, and 

 secondly the crystallographic and other physical characters, which 

 indicate more or less clearly the relations of individual species. 



