PHYSIOGRAPHY. 201 



Feldspar. 



Fig. 194. 



LaClayette and Loire in France. 



Sometimes there occurs a composition according to sev- 

 eral of these laws at once. Massive : composition granu- 

 lar, of various sizes ojf individuals, sometimes lamellar. 



1. Several distinct species were formerly included under the name of 

 Feldspar, and variously subdivided into subspecies and varieties. First, 

 those gray varieties which possess bright iridescent colors, were establish- 

 ed into a particular subspecies, under the name nt Labrador Feldsnar, 

 These are now known to comprehend varieties of Feldspar and of Labra- 

 dorite. The most transparent and pure varieties, generally in implanted 

 crystals, lining the walls of narrow veins in ancient rocks, were likewise 

 considered as a particular subspecies, and called JLdularia. It is made 

 up of Feldspar and Albite. The less transparent varieties were divided 

 into common and compact Feldspar ; the first of which contained, though 

 not exclusively, easily cleavable crystals ; the second, imbedded crys- 

 tals, having no distinct cleavage, and compound masses of small, or im- 

 palpable and strongly connected individuals. Common Feldspar con- 

 tains varieties of all the species enumerated above. From it, Clinkstone, 

 which is commonly a mixed mineral, and forms the mass of porphyry- 

 slate, was distinguished as a particular species ; so also was Variolite, 

 consisting of small globular masses, imbedded in a mixed rock. It has 

 not been exactly ascertained to what species Clinkstone and Variolite be- 

 long. Imbedded crystals of considerable degrees of transparency, in por- 

 phyry slate, occurring also in various other trachytic and volcanic rocks, 

 were called glassy Feldspar. Ice-spar occurs in white transparent 

 crystals, greatly resembling Adularia and glassy Feldspar, but implanted 

 in the drusy cavities of rocks ejected by Mount Vesuvius. In regard to 



