508 REPORT OF NATIONAL MUSEUM, 1890. 



in many mineral silicates, as, for instance, orthoclase. Granitic rocks 

 contain from 4 to 5 per cent, of potassium salts, which on their decom- 

 position become available for plant foods. As a chloride, potassium is 

 always found iu sea water, and as a nitrate forms the valuable natural 

 salt commonly called niter or saltpeter. 



Sodium. Symbol Na ; atomic weight 23: Sodium is never found free 

 iu nature, but its most common form is in combination with chlorine 

 forming common salt, an important ingredient of sea water. Combined 

 with silica sodium is an important element in many mineral silicates. 



Iron. Symbol Fe; atomic weight 56: Iron is the most abundant of 

 the heavy metals, and occurs in nature both free and combined with 

 other elements. In the free state it is found only to a limited extent in 

 basaltic rocks and meteorites, but in combination with oxygen it is one 

 of the most widely diffused of metals, and forms the coloring matter of 

 a large number of rocks and minerals. In this form, too, it forms the 

 valuable ores of iron known as magnetite and hematite. In combina- 

 tion with sulphur it forms the mineral pyrite FeS 2 . 



Manganese. Symbol Mn; atomic weight 55: Next to iron, mangan- 

 ese is the most abundant of the heavy metals. It occurs in nature 

 only in combination with oxygen, in which form it is associated in mi- 

 nute quantities with iron in igneous rocks or in the forms known min- 

 eralogically as pyrolusite, psilomelane and wad. As the peroxide of 

 manganese it occurs in concretionary forms scattered abundantly over 

 the floor of the deep sea. 



Barium. Symbol Ba; atomic weight 137 : Barium occurs in nature 

 combined with sulphuric acid, forming the mineral barite, or heavy 

 spar, or with carbonic acid forming the mineral witherite. 



(2) The minerals of rocks. 



A rock is a mineral aggregate; more than this, it is an essential por- 

 tion of the earth's crust, a geological body occupying a more or less 

 well-defined position in the structure of the earth, either in the form of 

 stratified beds, eruptive masses, sheets or dykes, or as veins aud other 

 chemical deposits of comparatively little importance as regards size 

 and extent. Having considered the elements which in their various 

 combinations go to make up the minerals, we will now consider the 

 minerals which go to make up the rocks. The collection, it should be 

 stated, is designed to show only those minerals commonly found fu rock 

 masses, or which make up any considerable portion of the mass of a 

 rock of any kind. The specimens are selected, not on account of their 

 beauty or fine crystallographic development, but ordinary forms, both 

 crystalline and massive, are shown in their principal varieties. 



The mineral composition of rocks, it may be stated, " is greatly sim- 

 plified by the wide range of conditions under which the commonest 

 minerals can oe formed, thus allowing their presence in rocks of all 

 classes aud of whatever origin. Thus quartz, feldspar, mica, the min- 



