Earth Study 



829 



Ward's Natural Science Establishment, College Avenue Rochester, N. Y. 

 These collections vary in number of specimens and price from one to two 

 dollars and are excellent. The teacher should have one or two perfect 

 crystals of quartz, feldspar and calcite. An excellent practice for a boy is 

 to copy these crystals in wood for the use of the teacher. 



The physical characteristics used in identifying minerals are briefly as 

 follows : 



1. Form. This may be crystalline, which shows the shape of the 

 crystals definitely; granular, like marble, the grains having the internal 

 structure, but not the external form, of crystals; compact, which is without 

 crystalline form, as limestone or flint. 



2 . Color. 



3. Luster or shine, which may be glassy like quartz, pearly like the 

 inside of a shell ; silky like asbestos ; dull ; or metallic like gold. 



4. Hardness or resistance to scratching, thus: Easily scratched with 

 the finger nail; cannot be scratched by the finger nail; easily scratched 

 with steel; with difficulty scratched with steel; not to be scratched by 

 steel. A pocket knife is usually the implement used for scratching. 



Forms of quartz crystals. 



QUARTZ 



Teacher's Story 



There is in the Cornell University Museum a great quartz crystal, a six- 

 sided prism several inches in thickness. One-half of it is muddy and the 

 other half clear, transparent and beautiful. The professor in charge, who 

 has the imagination necessary to the expert crystallographer, said to his 

 class: "This crystal was begun under conditions which made it cloudy; 

 then something happened, perhaps some cataclysm that changed all the 

 conditions around the half -grown crystal, and it may have lain a hundred 

 or a thousand years unfinished, when, some other change occurring, there 

 came about conditions which permitted it to resume growth, and the work 

 began again exactly where it was left off, the shaft being perfected even to 

 its six-sided pyramidal tip." And ever afterwards that crystal, half clouded 

 and half clear, remained in the minds of his pupils as a witness of the eternal 

 endurance of the laws which govern the growth of crystals. 



Quartz is the least destructible and is one of the most abundant materials 

 in the crust of the earth as we know it. It is made up of two elements 

 chemically united the solid silicon and the gas oxygen. It is the chief 

 material of sand and sandstones, and it occurs, mixed with grains of other 

 minerals, in granite, gneiss, and many lavas; it also occurs in thick masses 

 or sheets, and sometimes in crystals ornamenting the walls of cavities in the 

 rocks. Subterranean waters often contain a small amount of silica, the 

 substance of quartz, in solution; from such solutions it may be deposited in 



