MINERALOGY. 8 1 



hence those that show the very evddent basal cleavage, when revolved on 

 the stage of the microscope with only the lower Nicol fixed upon the 

 instrument, are light yellow when the vertical axis of the crystal is parallel 

 to the plane of vibration of the light, and very dark brown or black when 

 the cleavage, which corresponds to the lateral axes, is parallel to this 

 plane. Sections that show no cleavage are basal sections, and, as basal 

 sections of hexagonal minerals do not possess double refraction, they 

 show no dichroism. It is possible to confound biotite with basaltic 

 hornblende, since their colors and dichroism are much alike, though 

 the cleavage of mica is dtfferent. Between crossed Nicol prisms all 

 sections of biotite will be black when the cleavage corresponds with 

 the plane of vibration of either Nicol, since the cleavage corresponds 

 with an axis of elasticity. With hornblende this is not the case, and in 

 the larger number of its sections the point of maximum darkness will 

 be obtained when the cleavage makes a certain though not great angle 

 with the plane of the light. 



Biotite is constantly met with in our rocks, and in many cases it re- 

 quires no microscope to detect it; but in many other cases it is only 

 microscopic. 



Biotite is fusible before the blow-pipe ; but the ease with which it fuses 

 depends upon the amount of iron that it contains. Our black biotites in 

 the feldspar quarries are very rich in iron, and hence fuse without much 

 difficulty to magnetic globules. Some specimens impart the bright crim- 

 son color that is characteristic of lithia. There was one per cent, of 

 lithia in the biotite from Middletown that I analyzed. Some of the bio- 

 tite in the granites contains lithia; and the lithia-bearing varieties seem 

 to be much more easily fusible than those which contain none. 



If an ordinary cleavage piece of biotite, sufficiently thin to be translu- 

 cent, is put on the stage of the microscope, the ocular removed, and the 

 two Nicol prisms placed at right angles to one another, the field will be 

 traversed by a black cross, the arms of which do not alter their positions 

 on revolving the stage. The mica is thus easily seen to be uniaxial. 



55. Lepidomelane. 



From an examination of the analysis of biotite that precedes, it is 

 easy to see how readily a mineral so composed would alter. Such 



