MINERALOGY. 10$ 



usually contain some lithia, and arc referred by Rammelsberg to a sec- 

 ond division of the species. They contain no iron, and have a different 

 general formula, the difference being mainly produced by a higher 

 percentage of alumina. 



Beside the localities mentioned, tourmalines arc abundant in many of 

 our rocks in such quantities as to excite interest, though cabinet speci- 

 mens are not common. For example : all through the White Mountains 

 little tourmalines arc seen here and there scattered through the schists. 

 Sometimes they are very abundant, and of considerable size, and some- 

 times they are very small and sparsely disseminated. 



The power which tourmalines possess to polarize light depends simply 

 on the circumstance that the light only passes through the crystal par- 

 allel to the vertical axis, the vibrations at right angles thereto being 

 absorbed. This gives a most ready means for recognizing tourmalines 

 in rock sections, since those sections which are cut parallel to the prism, 

 when placed under the microscope with only the lower Nicol on the 

 instrument, are light colored when the longer axis of the crystal is par- 

 allel to the plane of vibration of the light, and almost black when placed 

 at right angles to it. Basal sections must therefore be nearly opaque, 

 and do not change on being revolved. Biotite, hornblende, and other 

 dichroic minerals possess perfect cleavages, while tourmaline has none. 

 Microscopic tourmalines are often seen in our rocks. 



6^. Andalusite [Al'^ .Si O']. 



This silicate is very often found in our rocks. It crystallizes in square 

 prisms of the orthorhombic .system, which vary in size from microscopic 

 bits to crystals an inch square. Their surfaces are never smooth, and 

 their edges are usually rounded. At times, crystals or portions of crys- 

 tals are pure, and possess a vitreous lustre ; but, on the other hand, 

 andalusite is more common, which presents itself as a mere hard, irreg- 

 ular spot in the rock, which becomes prominent on account of its greater 

 ability to withstand the weather. Moreover, the centres and surfaces of 

 the prisms often differ in their power of withstanding decay ; and hence 

 crystals are often found which have become hollow cylinders, a result of 

 the rotting out of the centre. Again : both surface and centre resist 

 while the rest gives way, and we then obtain a hollow cylinder with a 

 VOL. IV. 14 



