o 



22 STRUCTURE OF MINERALS. 



ity at Zinken, in Germany, 1000 cwt. of cry*tals of quartz 

 were taken above a century since. Thespfacts indicate im- 

 perfectly the scale of operations in the laboratory of nature. 

 The same process by which a single group, like that just alluded 

 to, has been formed, has filled numberless similar cavities over 

 various regions, and distributed the quartz material through 

 vast deposits in the earth's structure. The same powei 

 presides alike over the solidification of liquid lavas, and the 

 formation of a cube of salt, producing the crystalline grains 

 constituting the former, and the structure and symmetrical 

 faces of the latter. 



Constancy of Crystalline Forms, Each mineral may be 

 properly said to have as much a distinct shape of its own, as 

 each plant or each animal, and may be as readily distin- 

 guished by the characters presented to the eye. Crystals 

 are, therefore, the perfect individuals of the mineral kingdom. 

 The mineral quartz has a specific form and structure, as much 

 as a dog, or an elm, and is as distinct and unvarying as re- 

 gards essential characters, although, owing to counteracting 

 causes during formation, these forms are not always assumed. 

 In whatever part of the world crystals of quartz may be col- 

 lected, they are fundamentally identical. Not an angle will 

 be found to differ from those of crystals obtained in any part 

 of this country. The sizes of the faces vary, and also the 

 number of faces, according to certain simple laws hereafter 

 to be explained ; but the corresponding angles of inclina- 

 tion are essentially the same, whatever the variations or dis. 

 tortions. 



Other minerals have a like constancy in their crystals, and 

 each has some peculiarity, some difference of angle, or some 

 difference of cleavage structure, which distinguishes it from 

 every other mineral. In many cases, therefore, we have only 

 to measure an angle to determine the species. | Both quartz and 

 carbonate of lime crystallize at times in similar six-sided 

 prisms with terminal pyramids ; but the likeness here ceases ; 

 for the angles of the pyramids are quite^drflterent, and also 

 the internal structure. Idocrase and tin ore crystallize in 

 similar square prisms, with tejwrtinal pyramidal planes ; but 

 though similar in genexaHferai^each has its own character 

 i>tic angles of inclination between its planes, which angles 



What is said of the generality of the power of crystallization 1 "What 

 is said of the constancy of the crystalline forms and .structure, of minerals' 

 Explain by the mineral quartz, as an example. 



