MINERALOGY, 83 



and Springfield, are towns where large granitic veins have been worked 

 with profit for the mica, and the Grafton mica mines are still in very 

 successful operation. Alexandria, Orange, and Groton are other local- 

 ities. Much further reference to these occurrences of mica will be found 

 in the chapter on economic geology. 



Muscovite is orthorhombic, and the angle of its prism is 120°. The 

 crystals appear sometimes nearly hexagonal, on account of the develop- 

 ment of the brachy-pinnacoid. In Wilmot, large crystals, six inches in 

 diameter, are found, which by the combination indicated appear nearly 

 hexagonal. Gilmanton, New London, and Hinsdale, beside the mica 

 quarries already mentioned, furnish fine crystalline varieties of musco- 

 vite. This mica is usually colorless, or smoke color in thicker plates ; 

 but green and yellow muscovite is found at Bedford, rose-colored at Wal- 

 pole, green, white, and brown at Piermont, and green at Unity. All 

 shades of yellow result from alteration, and an opaque gold color is 

 often the last product of change. 



The association of minerals with muscovite in our granitic veins is very 

 interesting. Its union with biotite according to a definite rule has been 

 already mentioned under that species. The simplest case of that asso- 

 ciation is where plates of black and white mica are united by their edges, 

 as illustrated in Fig. 9 on PI. 2. The cleavages developed by a blow with 

 a sharp point are parallel in the two species, and, as shown by Reusch, 

 these cleavages are parallel to the prismatic faces of the two species. 

 The sides of the hexagon on the biotite may therefore be drawn, but 

 the prism of muscovite might be drawn with its long axis in three 

 different directions. If now we remove the ocular from the microscope, 

 on looking through the biotite with the Nicol prisms crossed, the strictly 

 uniaxial character of the mica will be seen by the clear black cross, the 

 arms of which cross one another in the centre of the field. (Biotite is 

 not always so strictly uniaxial.) Muscovite is orthorhombic, and the 

 vertical axis is the acute bissectrix ; and in our crystals the plane of the 

 axes appears always to be that of the macrodiagonal. If now, with the 

 ocular removed, we look at the muscovite, we see its two optic axes, and 

 the direction of the line connecting them is the longer diagonal of the 

 rhomb. These observations are represented in the figure ; a and b are 

 the cleavages produced by blows, and the appearances of the optic axes 



