322 NATURE STUDY REVIEW [8 :9— Dec, 1912 



known. Many minerals are represented by one specimen in 

 some museum and a great many more are never seen outside of 

 museums. The really common minerals number less than a 

 hundred. F. W. Clarke^ estimates the average mineralogical 

 composition of the igneous rocks which have been described, as 

 follows : 



Quartz 12.0% 



Feldspars 59.5% 



Hornblende and pyroxene 16.8% 



Mica 3.8% 



Other minerals 7.9% 



100.0% 



These figures are averages ;and do not indicate that any 

 rock specimen would have these minerals in the above propor- 

 tions. As we shall see later there are all variations possible 

 in the amounts of minerals in igneous rocks. When we stop 

 to think that igneous rocks constitute 95 per cent of the out- 

 er part of the earth we see at once how abundant these min- 

 erals are. 



In some localities a collection of 25 minerals and rocks would 

 be all that could be collected, while in other regions fifty or more 

 common minerals could be collected very easily. By following 

 the descriptions given below any teacher should be able to de- 

 termine the common minerals which are found in his locality. 

 The chemical composition is written out and the chemical form- 

 ula is also given when a definite one is known. The common 

 uses for each of the minerals are also given and they should be 

 told to the scholar. While the minerals in the list are very wide- 

 spread in occurrence there are many localities where only a few 

 can be found. The descriptions are given in the order of their 

 hardness, the softest first. This will aid in locating the probable 

 descriptions of the mineral. 



The method of using the table is as follows : Determine 

 the hardness of the mineral as carefully as possible, being care- 

 ful to use fresh material in doing so. Then locate it in the table 

 by means of the first column. If there are one or more minerals 

 that have the same hardness make use of the other physical 

 properties of the mineral, such as color, structure, streak, luster, 

 etc., in determining it. In a short time the especial features of 

 each mineral that will distinguish it from the remainder will be 

 remembered. vSome of these especial features have been sug- 



2Clarke, Ihilletin 491, U. S. Geological Survey, p. 31. 



