292 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1920. 



of iron are apparently unknown in nature, though of nickel there 

 are some very rare minerals of this character. It will be seen that 

 such compounds (sulphides, arsenides, etc.) of the petrogenic ele- 

 ments are all of those of rather high atomic weight and in the groups 

 of highest valence, especially common in the triad group iron-cobalt- 

 nickel. 



Turning to the metallogenic elements, we find that many of them 

 exist in nature uncombined, notably copper, silver, gold, mercury, 

 arsenic, antimony, bismuth, sulphur, selenium, tellurium, and the 

 metals of the platinum group. Native zinc, lead, tin, and tantalum 

 are also reported, but in some cases doubtfully. 



As minerals the oxides of these elements either do not exist (as 

 of gold and the platinum metals), are of extreme rarity, or are cer- 

 tainly or almost certainly of secondary origin, as those of copper, 

 mercury, zinc, arsenic, and antimony. Tin oxide, the common ore of 

 this metal (cassiterite), is an apparent exception, but it would seem 

 to be possible that, in some cases at least, it is of secondary origin, 

 a sulphide being the primary compound. 



Primary silicates of the metallogenic metals are very rare. There 

 are none of gold, silver, mercury, thallium, tantalum, tungsten, or 

 the platinum metals. Silicates of copper and zinc are quite common, 

 but are in all cases almost undoubtedly of secondary origin. There 

 are, however, silicates (possibly primary) of tin, lead, and bismuth, 

 but they are mineral rarities, and many mineralogical museums and 

 collections have no specimens of them. 



No fluorides of any of the metallogenic elements are known as 

 minerals, but insoluble chlorides and oxychlorides of copper, silver, 

 mercury, and lead are known, though rare. On the other hand, as 

 native bromides and iodides we know only those of copper, silver, 

 mercury, and lead — all metallogenic elements. 



The typical, and by far the most abundant, native compounds of 

 all these metallogenic elements then are the sulphides, arsenides, 

 antimonides, selenides, and tellurides, with the complex sulphosalts. 

 These form the main, and in some cases the only, sources of most 

 of the metals. Indeed, of gold, mercury (except the common sul- 

 phide, the secondary oxide, a chloride, and two doubtful iodides), 

 and thallium (except a rare sulphide), the only native compounds 

 known are selenides and tellurides; and conversely, the only native 

 selenides and tellurides known are of copper (rare), silver, gold, 

 mercury, thallium, lead, and bismuth, except that there is a very rare 

 nickel telluride. Oddly enough, the only native compounds known 

 of the platinum metals are ruthenium sulphide and platinum arsenide, 

 no selenides or tellurides of these being known. 



Returning to the intermediate interlocking meander zone, it may 

 be well to point out some features that show to which of the two 



