18 PHYSIOGRAPHY. 



Andalusite. 



their lateral edges. It is not common to find the edges of the bases 

 equal, as in fig. 12 ; two of the sides being more frequently unduly ex- 

 tended, as in figs. 13 and 14. Cross-sections made in different parts 

 of one crystal sometimes afford the different appearances of figs. 12, 

 13, and 14. But the more common mode of aggregation is represented by 

 * figs. 15, 16 X 17 and 18, which results from the greater or less replacement 

 of the angles that are adjacent in the composition. Rarely, we are pre- 

 sented with a parti-colored aspect, as in figs. 19 and 20, the inner por- 

 tion a exhibiting a milk-white color, while the exterior &, is of a green- 

 ish grey color. The hardness of these crystals varies from 3-D to 7-5, 

 according to the preponderance of Andalusite in their composition. 

 Those which possess the hardness of 7-5 have the color, lustre and cleav- 

 ages of Andalusite. The circumstance which determines to the forma- 

 tion of these twin-crystals pertains apparently to the gangue ; for the 

 Macle occurs only in clay slate, while a similar aggregate, varying in 

 hardness from 3-0 to 7-5, is found imbedded in single crystals of the form 

 of fig. 12, in veins of Quartz, at the same locality. Before the bjow- 

 pipe, Macle affords a little moisture without suffering any alteration. In 

 a white heat, its color becomes white, but it does not undergo the slight- 

 est fusion. With borax, it is with much difficulty dissolved into a trans- 

 parent glass. According to a recent analysis of LANDGREBE, it con- 

 sists of Silica 68-497, Alumina 30-109, Magnesia 1-125, Water and Car- 

 bon 0-269 ; and another by JACKSON, of Silica 33-0, Alumina 61.0, pro- 

 toxide of iron 4-0, and Water 1-50. 



2. Alone, before the blowpipe, it whitens in spots, but does not melt. 

 With borax, it dissolves with great difficulty into a transparent and col- 

 orless glass. 



3. Analysis. 

 By VAUQTJELIJY, By BRANDES, 



from Spain. from Tyrol. 



Silica . . 3246 . . . 34-000 



Alumina . . 52-24 . . . 55-750 



Potash . . 8-10 . . . 2-000 



Oxide of iron . . 200 . . . 3-375 



Loss . . 6-00 . . . 0-000 



Lime 2-125 



Magnesia 0-375 



Oxide of manganese 3-625 



Water 1-000 



4. Crystals of Andalusite are found imbedded in mica slate, or im- 

 planted in the cavities of rocks forming irregular beds or nodules in gra 



