52 STRUCTURE OF MINERALS. 



the required angle is 121° 18' ; if this line marks 125° 30' 

 the required angle is 125° 48'. 



Some goniometers are furnished with a small polished re- 

 fleeter, attached to the foot of the instrument below the part 

 s, q, which is placed at an oblique angle so as to reflect a bar 

 of the window. The reflected bar then answers the purpose 

 of the line drawn below the window, (or on a slate,) and "s 

 more conveniently used. 



Other modes of adjustment for the crystal, are also used 

 but they will explain themselves to the student acquainted 

 with the above explanations, and need not here be dwelt 

 upon. 



MASSIVE MINERALS, OR IMPERFECT CRYSTALLIZATIONS. 



Massive or imperfectly crystallized minerals either consist 

 of fibers or minute columns, of leaves or laminae, or of grains : 

 in the first, the structure is said to be columnar ; in the 

 second, lamellar ; in the third, granular. We have a familiar 

 example of the lamellar structure in slate rocks and many 

 minerals that occur in masses made up of separable laminae. 

 The fibrous or columnar structure is common in seams of 

 rocks, and sometimes in incrustations covering exposed sur- 

 faces ; the material of the seam or crust is made up of mi- 

 nute fibers or prisms closely compacted together, produced 

 by a rapid crystallization on the supporting surface. The 

 granular structure is well seen in loaf sugar and statuary 

 marble. 



1. Columnar Structure. The following are explana- 

 tions of the terms used in describing the different kinds of 

 columnar structure. 



Fibrous ; when the columns are minute and lie in the 

 same direction ; as gypsum and asbestus. Fibrous minerals 

 very commonly have a silky luster : a fibrous variety of 

 gypsum, and one of calc spar, have this luster very strongly, 

 and each is often called satin spar. 



Reticulated ; when the fibers, or columns, cross in various 

 directions, and produce an appearance having some resem- 

 blance to a net. 



Stellated ; when they radiate from a center in all direc- 

 tions, and produce a star-like appearance. Ex. stilbite, 

 gypsum. 



What kinds of structure exist in massive minerals 1 Explain the dif- 

 ferent varieties of columnar structure, fibrous ; reticulatei, &c. 





