. 175. OF SURFACE. 237 



These striae are produced in the following way. Instead 

 of the faces of the pyramid, P, 2, Fig. 73., which in 

 the perfect combination would continue Avithout being 

 interrupted from the edges of combination to the apices, 

 the faces of the prism r, r' re-appear. These faces, how- 

 ever, do not reach very far, but are again exchanged 

 for the faces of the pyramid, which in their turn must yield 

 to the faces of the prism, and this alternately, till at last 

 the faces of the pyramid meet in the apices, as it is repre- 

 sented in Fig. 73- If now we suppose the faces altogether, 

 and more particularly those of the pyramid, to become very 

 narrow, we may form an idea of those delicate striae, which 

 are so often met with in nature. It is, however, not very 

 rare, to be able to observe immediately the formation of 

 the striae, on the large scale now described, which per- 

 fectly confirms the above explanation. 



It is evident, that the striae thus produced must be pa- 

 rallel to the edges of combination between the faces of the 

 pyramid arid those of the prism ; because the faces of those 

 forms alternate in their edges of combination. 



The situation of the faces (viz. those of the prism), is 

 indeed altered by these striae ; yet this is inconsiderable if 

 the striae become very delicate, and the more so, if those 

 faces which belong to the opposite apex of the pyramid, 

 likewise alternate with the former, and exercise their in- 

 fluence upon the streaking, as may be seen in Fig. 74. It 

 will require some attention if we have to apply measuring 

 instruments to crystalline faces disfigured by these striae. 



Another very remarkable example of striated faces oc- 

 curs in hexahedral Iron-pyrites, in the combination of the 

 hexahedron, and of the hexahedral pentagonal-dodecahe- 

 dron. Vol. II. Fig. 1G5. In this example, the striae are 

 parallel to the edges of combination ; hence they are parallel 

 to each other upon parallel faces, and perpendicular to each 

 other upon such faces as are not parallel. If, instead of the 

 hexahedron, the form combined with the dodecahedron is a 

 trigrammic tetragonal-icositetrahedron, whose characteristic- 

 angle is equal to the characteristic edge of the dodecahe- 



