CRYSTALLOGRAPHY. 23 



admit of no essential variation. Upon this character, the 

 constancy of crystalline forms, depends the importance of 

 crystallography to the mineralogist. 



FUNDAMENTAL FORMS OF CRYSTALS. 



The forms of crystallized minerals are very various. To 

 he eye there often seems to be no relation between different 

 crystals of the same mineral. Yet it is true that all the va- 

 rious shapes are modifications according to simple laws of 

 a few fundamental forms. There is perhaps no mineral 

 which presents a greater variety of form than calc spar. 

 Dog-tooth spar is one of its forms ; nail-head spar, as it is 

 sometimes called, is another ; the one, a tapering pyrimadal 

 crystal, well described in its name, the other broad and thin, 

 and shaped much like the head of a wrought-nail. Yet both of 

 these crystals and many others are derived from the same fun- 

 damental form. After a few trials with a knife, the student 

 will find that slices may be readily chipped off from the crys- 

 tals of this mineral in three directions ; and the process will 

 obtain a solid from each, the one identical with the other in 

 its angles. They consequently have the same nucleus or 

 fundamental form. 



The fundamental forms are those from which all the other 

 torms of crystals are derived. The derivative forms, are 

 called secondary forms, and their planes, secondary planes. 



The number of fundamental forms indicated by cleavage, 

 is thirteen. They are either prisms* octahedrons or dode- 

 cahedrons. 



The prisms are either four -sided or six-sided. The prisms 

 are denominated right prisms, when they stand erect, and 

 oblique prisms, when they are inclined. Figures 4, 5, 7, 8, 

 are right prisms, and figures 12, 14, are oblique prisms. 

 The sides in each case are cal]ed lateral planes, and the 

 extremities bases. 



An octahedron^ has eight sides, and consists of two equal 



How do the crystals of different minerals differ 1 Mention exam- 

 ples. What is said of the forms of crystals of the same mineral ? 

 What is understood by fundamental forms 1 What by secondary forms 

 or planes 1 How many fundamental forms are there 1 What kinds of 

 prisms are there 1 Explain the terms lateral planes and bases. 



* Any column, however many sides it may have, is called a prism. 

 t From the Greek okto, eight, and hedra, face. 



