FELTING 337 



cleavage planes, indicating the junction of twin-crystals; transparent microscopic 

 sections viewed under polarised light, exhibit coloured bands corresponding with these 

 twin-lamellse. The plagioclases comprise all species of felspar, except orthoclaso and 

 sanidino. 



The principal economic value of the felspathic minerals depends on their applica- 

 tion to porcelain manufacture, partly as* a constituent of the paste itself, but chiefly 

 as a glaze. From the decomposition of the felspar in certain granites, the well-known 

 kaolin, or China clay, is commonly supposed to result. _ The felspathic minerals are 

 also employed as manures, the alkalis present rendering them especially vahiable. 

 See CLA.Y ; POTTERY ; MANURES. 



FZIiSPATZXIC. Of or belonging to felspar. 



FEXiSPATHZC ROCKS. (Roches feldspathiques, Fr. ; Feldspathgesteine,^ Q-er.) 

 Rocks in which felspar forms an essential and frequently the principal constituent. 

 Felstone, felsite, or corncan, is a rock consisting almost exclusively of a compact 

 orthoclase, associated with a small amount of free silica, and much resembling in its 

 general characters certain forms of quartz, whence it has unfortunately been termed 

 pctrosilex. From all varieties of quartz, it may however be readily distinguished by 

 its fusibility, and hence it was called by Werner fusible hornstone. A felstone 

 occurring with the magnetic iron-ore of Dannemora retains its Swedish name of 

 Halleflinta ; and the leclite of Dr. Clarke appears to be a very similar rock. The 

 name of adinole has been applied by Beudant to a rock closely related to felstone, but 

 consisting of compact albite-felspar. When distinct felspar-crystals are developed in 

 a compact base of felstone, the rock becomes &felspar-porphyry; and when these are 

 accompanied by quartz it forms a quartziferous-porphyry or elvanite. Of these rocks, 

 formed of mixtures of felspar and quartz, several varieties are recognised ; such as 

 eurite, aplite, granulite, and leptynite. Pegmatite is a very similar rock, often known as 

 graphic granite from the peculiar disposition of the quartz-crystals in the felspar-base. 

 Such a rock may be termed a binary granite, since it requires only the addition of 

 some micaceous mineral to convert it into a true granite. The granitic rocks have 

 been divided into numerous varieties founded either on peculiarities of texture, such 

 as gneiss, which is commonly regarded as a schistose granite; or on variations in 

 composition, arising generally from the presence of certain accessory minerals : thus, 

 the bercsite from Bersof in the Ural Mountains is a granite containing iron-pyrites. 

 In certain granitic rocks, the mica is replaced by other minerals giving rise to several 

 distinct varieties : in the Cornish luxullianite, for example, it is replaced by schorl, 

 and in the so-called protogine of the Alps by talc ; whilst in syenite its place is 

 supplied by hornblende. In the zircon-syenite of Norway, zircon occurs as an 

 accessory constituent ; and to this rock the Siberian miascite is closely related. On 

 the disappearance of the quartz in syenite, a rock is obtained consisting of felspar and 

 hornblende, and commonly termed greenstone; of which rock diorite, aphanite, timazite, 

 and amphibolite, may be mentioned as varieties. A singular concretionary greenstone 

 from the Island of Corsica is known as Napoleonite or orbicular diorite, and sometimes 

 very improperly as Corsican granite. 



The rocks passing under the names of basalt, doleritc, anamesite, and melaphyre, all 

 bear a greater or less resemblance to greenstone, but are distinguished by containing 

 the more basic felspars, labradorite or oligoclase, associated always with augite, and 

 often with various other minerals, such as olivine, magnetic iron-ore, &c. Many of 

 these rocks are popularly united under the general name of trap. 



Another series of felspathic rocks is formed by those volcanic products of which 

 trachyte is the type. This rock consists almost exclusively of felspar, and often 

 becomes porphyritic by the presence of crystals of sanidine. Among the varieties of 

 trachyte may be noticed the andesite of the Andes, and the domite of the Puy-de- 

 Dome. Ehyolitc is a highly-silicated trachytic rock. A compact fissile rock allied to 

 trachyte is termed phonolite or clinkstone, in allusion to the ringing sound which it 

 emits on being struck. 



When trachytic rocks have been completely fused they pass into a vitrified condition, 

 forming a variety of obsidian or volcanic glass. A capillary form of obsidian from the 

 Sandwich Islands is known as Pele's hair. When obsidian loses its vitreous texture 

 it passes into pitchstone ; and when presenting a concretionary structure it is termed 

 pearlstone. Pumice is simply a cellular or fibrous form of felspathic lava. 



Special descriptions of such felspathic rocks as are of any practical importance will 

 be found under their respective headings. 



FELT. (Feutre, Fr. ; Filz, Ger.) A material formed by working into a sheet, 

 wool or hair, upon the principle of their matting together. 



FEITIETG- (Feutragc, Fr. ; Filzcn, Ger.) is the process by which loose flocks 

 of wool, and hairs of various animals, as the beaver, rabbit, hare, &c., are mutually 

 interlaced into a compact textile fabric. The first step towards making felt is to mix, 



VOL. II. Z 



