EARTH'S CRUST — WASHINGTON". 291 



mides, or iodides, but only seldom and exceptionally do these occur 

 as (primary) silicates, oxides, fluorides, or chlorides. They are fre- 

 quently met with as the "native" elements. There are, it is true, 

 some exceptions to these statements (as with iron, which forms three 

 common sulphides, and with tin which occurs mostly as the oxide) ; 

 but taken broadly, and as applying to the two several groups as a 

 whole, the distinction seems to be valid. 



The oxides of many of the electropositive petrogenic elements 

 are known to occur as minerals; those, that is, that are stable and 

 are not readily soluble. They include periclase (MgO), corundum 

 (Al 2 O s ), quartz and tridymite (Si0 2 ), rutile (Ti0 2 ), ilmenite 

 ((Fe, Ti) 2 3 ),chromite (FeO.Cr 2 O s ), pyrolusite (MnO,) and other 

 oxides of manganese, hematite (Fe 2 O s ) and magnetite (Fe 3 4 ). 

 All of the electropositive petrogenic elements form silicates, and, 

 indeed, they form the overwhelming majority, certainly 99.9 per cent 

 by weight of all known silicates. Besides the simple silicates are 

 borosilicates, fluosilicates, titanosilicates, and zirconosilicates, all of 

 them salts of petrogenic elements. A few sulphosilicates are known, 

 but they are very rare, and there are no known arseno-, antimono-, 

 seleno-, or tellurosilicates. 



Fluorides and chlorides of sodium, potassium, ammonium, mag- 

 nesium, calcium, aluminum, cerium, iron, and manganese are known 

 as minerals, and some of them are very common, as Nad, KC1, and 

 CaF 2 . On the other hand, neither bromides nor iodides of these 

 elements occur as minerals, though there is an excessively rare cal- 

 cium iodate. Fluorine replaces hydroxyl in several silicates, as in 

 topaz and chondrodite, and it is also present in small amounts in 

 hornblendes and micas, while chlorine is present in small amount in 

 some silicate minerals, as those of the sodalite and scapolite groups. 

 Until we reach vanadium, with atomic weight 51 and atomic number 

 25, no sulphides occur as minerals, except calcium sulphide, which 

 occurs as a rare mineral (oldhamite) but only in a few meteorites. 

 A very rare vanadium sulphide, found only in one locality, an ex- 

 tremely rare chromium-iron sulphide, occurring only in a few me- 

 teorites, and a rare terrestrial manganese sulphide are known. No 

 arsenides, selcnides, or tellurides of these elements, or of those pre- 

 ceding them in atomic number, are known. With the iron group, 

 we find sulphides very common, the sulphides of iron, pyrite, mar- 

 casite, and pyrrhotite, being common minerals, and sulphides of 

 nickel and of iron and nickel, as well as their arsenides, are wide- 

 spread ore minerals. Sulphides and arsenides of cobalt are also 

 fairly common. The sulphide of molybdenum is the only usual min- 

 eral of this element, though a few other minerals containing it (as 

 secondary molybdates) occur. Selenides, tellurides. and antimonides 



