INTRODUCTIOX. xiii 



In the first the axes connect the centres of opposite faces. 



In the second the vertical axis connects the centres of the basal faces, and 

 the lateral axes connect the centres of the opposite lateral edges. 



In the octahedron with a rhombic base the axes, as before, connect the 

 opposite solid angles. 



4. Oblique System. — This has three unequal axes. The vertical axis is 

 inclined to one of the lateral axes, and at right angles to the other — the two 

 lateral axes being also at right angles to each other. 



It comprises the right rhomboidal and the oblique rhomboidal prisms. 

 In the first the axes connect the centres of opposite faces. 

 In the second the vertical axis connects the centres of the bases, and the 

 lateral axes the centres of the opposite lateral edges. 



5. Anorthic System. — In the Anorthic System there are three unequal 

 axes, all intersecting each other obliquely. 



It comprises the oblique rhomboidal prism. 



6. Hexagonal., or Rhombohedral. — In this system there are three equal 

 lateral axes, intersecting at an angle of 60°, and one vertical axis at right 

 angles to them. 



It comprises the hexagonal prism and the rhombohedron. 



In the former the vertical axis connects the centres of the bases, and the 

 lateral axes the centres of the opposite lateral edges, or of the lateral faces. 



In the latter the vertical axis connects two of the solid angles diagonally 

 opposite, and the lateral axes opposite lateral edges. 



The student will derive great assistance in investigating the primary forms 

 of crystals and their modifications if he make a series of models for himself. 

 Drawings of these, which can be cut out in one piece, and after being stuck 

 on cardboard admit of being fastened together with a very slight degree of 

 trouble, answer the purpose extremely well, and are sold in Germany at a 

 very cheap rate. 



Besides occurring singly, crystals are sometimes found in twins or in macles. 

 In that case they are divided into two groups. 1st. Those in which the 

 crystals are united in such a way that the axes of the two separate crystals, 

 so united, are parallel to each other ; and 2nd, those in which the axes are 

 oblique or inclined to one another. 



In other instances minerals, instead of crystallizing in the forms which are 

 properly their own, assume pseudomorphous forms ; that is to say, forms be- 

 longing to some other kind of mineral. This may have happened in several 

 ways. Either the original mineral may have been entirely removed and the 

 newer one deposited in the cast (or the mould) of that which has disappeared, 

 or the original mineral may have been gradually removed atom by atom, 

 and for every particle so carried away portions of another mineral sub- 

 stituted. 



" Pseudomorphous crystals are distinguished, generally, by their rounded 

 angles, dull surfaces, and often granular composition. They either have no 

 cleavage, or the cleavage is wholly different in direction from that of the 

 mineral imitated. Their surfaces are frequently drusy, or covered with 

 minute crystals. Occasionally the resemblance to real crystals is so perfect, 

 that they are distinguished with difficulty."* 



* Dana's Mineralogy, vol. i. p. 136. 



