PHYSICAL PROPERTIES 



267 



often serve fur the identification of /ones or individual fnn-. 

 They are of two varieties; those caused by alterations of growth, 

 between two crystal forms and therefore parallel to their inter- 

 section or edge. :is the horizontal st nation* on the prism faces of 

 quarts. 



When the st rial ions are very marked, the crystal is said to be 

 furrowed. Twinning striations differ from oscillatory striutions, 

 or those caused by the alterations in growth between two form- in 

 that they represent the composition plane between twins which 



FIG. 380. Asbestos. Thatford, Quebec, Canada. 



penetrate and pass through the body of the crystal. They will 

 therefore appear on cleavage pieces as well as on crystal faces, 

 either singly, as in the Alaska epidotes, or often repeated as parallel 

 lines, as in the plagioclases. 



Vicinal faces. It has often been observed that each face of a 

 simple form, which may be represented by parameters or in- 

 dices of normal value, as the cube or octahedron in the isometric 

 system, is replaced by a low and very flat pyramid. The number 

 of faces of the pyramid will depend upon the symmetry of the face 

 replaced in the cube four and in the octahedron three. These 

 flat vicinal faces intersect the axes at long distances and their 

 indices, contrary to the general rule of simple indices, are large and 

 indefinite, though the faces conform to the symmetry of the type 



