PHYSICAL l'K<>l'l.i;rii.> 



273 



Botryoidal. When in rounded inv;ul;ir masses, usually with a 

 smooth surface and an internal radiated structure, as prehnite, 

 Fig. 389. When in forms resembling kidneys, often termed reni- 

 fonn, :t> M>me hematites. 



Stalactitic. Minerals are often formed by the evaporation of 

 solutions dripping from the walls and ceilings of caves; the solids 

 are deposited in the form of icicles or stalactites, as calcite, mala- 

 ( hite, or limonites, Fig. 390. Stalactites are often long, slender, 

 and hollow, the solution flowing down the hollow tube and deposit- 



FIQ. 391. Stalagmite of Calcite. 



ing the mineral substance only on evaporating at the lower end. 

 Such stalactites are strawlike. 



Stalagmites. Are the reverse of stalactites, or the structure 

 which forms on the floor of caves under the drip from a stalactite, 

 Fig. 391. 



Mammillary. When the crusts are composed of rounded masses 

 or nipple-like structures. 



Other terms in common usage in the description of minerals will 

 need no particular explanation. 



Color of minerals. The outward appearance of minerals, due 

 to other causes than form or structure, is caused by light, or color 

 and luster. The color of a specimen when opaque is caused by re- 

 flected light wholly ; when transparent, to transmitted light in 

 combination with reflected light. When white light falls upon a 

 mineral, some rays are absorbed and the complementary color is 

 T 



