416 



Geognosy. 



MINERALOGY. 



Gnciis, 



Name. 



Constitu- 

 ent parti. 



Imbedded 

 minerals. 



Kmda of 

 gneiss. 



Topaz rock is very distinctly stratified, and the strata 

 are of considerable thickness. It rests upon granite, 

 and is covered by clay-slate. 



It has hitherto been found only near to Auerbach, 

 in Voightland, in Saxony, where it forms a rock named 

 Schneckenstem, which was formerly of considerable ex- 

 tent, but has been much diminished by the operations 

 of the miners in procuring topazes. 



The inconsiderable extent of this mass prevents our 

 viewing it as a distinct species of mountain rock. 



II. Gneiss. 

 Gneiss. Werner. 



Gneiss and Granite Veine. Saussure. 

 Gneiss. Kirtvan. Jameson. 



1. Name. The name Gneiss is of Saxon origin, and 

 was applied by miners in the vicinity of Freyberg, to 

 the decomposed stone that forms the walls of their me- 

 talliferous veins, Henkel describes gneiss as an indu- 

 rated stone, mixed with steatitical and clayey matter ; 

 but Werner ascertained that it was a compound of fel- 

 spar, quartz, and mica. 



2. Constituent Parts. This rock, like granite, is a 

 compound of felspar, quartz, and mica ; but it contains 

 more mica than granite. It is granular in the small, 

 and slaty in the large ; hence it is said to be granular- 

 slaty. The granular felspar and quartz form plates, 

 -which are separated from each other by the mica. 



Felspar, although the predominant mineral, is still 

 in less quantity than in granite. The felspar is usual- 

 ly greyish, yellowish,and reddish.white; and sometimes 

 so much altered that it appears earthy. The mica is 

 most commonly grey, which passes through various 

 shades into blackish-grey. The quartz is almost always 

 greyish-white, and generally in smaller grains than 

 the felspar. 



3. Imbedded Minerals. Besides felspar, quartz, and 

 mica, it sometimes contains schorl ; more rarely garnet, 

 and also hornblende. The schorl occurs more rarely, 

 and in less quantity, than in granite : but the garnet is 

 more frequent and abundant than in granite. 



4. Kinds of gneiss. There are three principal kinds 

 of gneiss, of which we shall now give short descrip- 

 tions. 



( I .) In this kind the mica occurs in small quantity : 

 the scales of mica, although separated from each other, 

 are arranged in parallel ranges, and the rock breaks 

 in a direction conformable with these. It is the par- 

 allelism of the ranges of mica which distinguishes this 

 kind of gneiss from granite, because its slaty structure 

 is very indistinct, and the quantity of felspar is nearly 

 the same as in granite. The quartz and the mica form 

 each separate layers ; those of the felspar are thicker, 

 and such varieties, when broken across, have a ribbon- 

 like aspect. Sometimes the quartz, in place of being 

 disposed in layers or plates in the felspar, is in small 

 parallel rods or bars ; and when the rock is cut per- 

 pendicular to their direction, it appears not unlike pe- 

 trified wood. 



(2.) This, which is named common gneiss, consists of 

 small layers, or lenticular plates, composed of grains of 

 felspar and quartz, placed over each other, and sepa- 

 rated by layers formed of scales of mica. It is some- 

 times glandular, or contains balls of quartz, or of com- 

 pounds of quartz and felspar, or of mica. This va- 

 riety has been confounded with conglomerate. The 

 island of Fetlar, one of the Shetlands, affords an ex- 

 ample of this variety. 



(3.) This, the third variety, is very slaty, and very Geognosy, 

 micaceous. The scales of mica, from their smallness, V- ""Y"^ 

 appear indistinct, and form continuous plates. The 

 felspar and quartz are in very small grains, and are 

 sometimes so enveloped in the mica that it is difficult 

 to distinguish them. It is also sometimes glandular, 

 and in some instances almost an aggregation of balls 

 of mica. 



The gneiss which passes into granite belongs to the 

 first variety, as that which passes into mica slate does 

 to the third. 



5. Stratification. It is distinctly stratified, and the stratiflca- 

 strata are parallel with the slaty structure. But when tion. 

 the beds rest upon granite, they sometimes follow all 



the sinuosities of the irregular surface of that rock, 

 and form the mantle-shaped Gratification ; in other 

 cases the stratification is saddle-shaped, or it does jiot 

 appear affected by the granite, the strata passing with- 

 out change of direction from one mass of granite to 

 the other. 



6. Decomposition. This rock, like granite, decays Decompo- 

 on exposure to the atmosphere, but the decomposition sition. 



is in general more rapid. The felspar is at first chang- 

 ed into kaolin, and, owing to the greater abundance of 

 mica, the disintegration of the mass is more rapidly 

 effected. Hence it is that this rock does not occur so 

 often in great isolated blocks as granite ; and hence 

 also it is that mountains of gneiss are often less sharp 

 in their outline than those of granite, that their 

 summits are generally roundish, and that they rarely 

 shoot into needles, or are formed into denticulated 

 ridges. 



Sometimes the decomposition, in penetrating the 

 gneiss, loosens the adherence of the parts ; the folia 

 are then easily separated by the finger, and the mass 

 appears as if rotten. 



7. Foreign beds. These beds are more considera- Foreign 

 ble and more numerous in gneiss than in granite, beds. 

 The following may be enumerated. 



Limestone It is generally highly crystallized. Oc- Limestone. 

 curs in Aberdeenshire, Perthshire, and other parts of 

 Scotland; and on the continent of Europe, in the 

 Pyrennees, in Dauphiny, &c. 



Trap. -This rock, in the form of hornblende- rock, Trap, 

 hornblende-slate, and greenstone, occurs in beds, and 

 in imbedded masses, and is often very much inter- 

 mixed with the gneiss. When the gneiss abounds in 

 hornblende, it is named hornblendic gneiss. 



Porphyry. Beds, imbedded masses, and veins of Porphyry, 

 porphyry, sometimes of great magnitude and extent, are 

 not unfrequent in gome gneiss districts. Perthshire, 

 Aberdeenshire, and Inverness-shire, affords fine ex- 

 amples of porphyry in beds, imbedded masses, and 

 veins. 



Compact and granular felspar. The white stone. Compact 

 Weisstein of Werner. This rock sometimes occurs in granular 

 layers, which are not more than a few inches thick ; felspar. 

 in other instances in beds, so thick as to form whole 

 mountains. The felspar is white, and is very fine 

 granular, like dolomite : it contains numerous grains 

 of red garnet, even grains of quartz, and sometimes 

 scales of white mica. In short it is to be considered 

 as a granular felspar, generally containing scales 

 of mica, with some grains of quartz, and of other mi- 

 nerals. It forms beds and whole hills in Saxony, 

 Moravia, and Sweden. 



Quartz Rock. This, rock occurs in great beds, and 

 sometimes also in veins in gneiss districts. 



