MINERALOGY. 99 



to ten inches in diameter are not uncommon. The forms of the crys- 

 tals are simple. They are rarely perfect, but their great size makes them 

 interesting. The feldspar of these quarries is not economized, though 

 similar veins in Connecticut, where the mica is of a poor and unmarket- 

 able quality, are worked for the orthoclase alone. The feldspar is valu- 

 able for the manufacture of porcelain. 



In our granitic rocks orthoclase forms crystals of considerable size, 

 producing porphyritic rocks. In these cases the crystals are very com- 

 monly twins, recognizable macroscopically as such by the different 

 reflections of light from the two sides of the crystal, the cleavage faces 

 of which are differently inclined. Crystals of orthoclase, freely devel- 

 oped in cavities, are twinned according to one of four different methods ; 

 but ingrown crystals, with which we have mostly to deal, are almost 

 always what are called Carlsbad twins. These twins are formed as if 

 the crystal were divided in two by a plane parallel to the clino-pinna- 

 coid, and then one half revolved i8o° about an axis perpendicular to the 

 ortho-pinnacoid, and the two parts united together again, or grown into 

 one another. The clino-pinnacoid, by the large development of which 

 ingrown crystals are commonly flattened, is therefore the composition 

 face, but not the twinning plane. Examples of ingrown Baveno twins, 

 though uncommon, are not unknown; and a crystal of this nature is 

 represented in Fig. 4 on PI. 10, and is described under the head of 

 quartz porphyry. 



Orthoclase in granites, sienites, porphyries, etc., is white, flesh-colored, 

 or red. In our oldest gneisses, its crystals are flattened and rounded, 

 and surrounded by black mica, which gives them the appearance of eyes, 

 from which these gneisses have gained the name of augen gneiss. The 

 appearance of the porphyries is much modified by the color and lustre 

 of the feldspar. They are light in color when it is white and opaque ; it 

 sometimes makes them red, and when the feldspar is colorless and trans- 

 parent they are nearly black. The latter is the character of some of the 

 porphyries about Albany, and these rocks, when held in a proper posi- 

 tion, exhibit a beautiful play of colors that proceeds from the feldspar. 

 This play of colors is most characteristic of labradorite; but in our state 

 there is no labradorite that exhibits such a beautiful opalescence as this 

 orthoclase. This rock, if polished, would make a most beautiful orna- 



