414 



Geognoy. of the mica, Mont Blanc, and the surrounding moun- 



"""Y"^ tains, are formed of protogine. It is named protogine, 



(primccvi,) because Jurine, the author of the name, con- 



iders it as of very old formation. Daubuisson proposes 



to name it simply talcose, or steatitic granite. 



Sienite, 



Sienite. Sienite is a granite, in which the mica is generally 



replaced by hornblende ; not always, as some varieties 

 contain also mica. Werner says it is a granular aggre- 

 gated rock, composed of felspar and hornblende, with 

 occasional grains of quartz and scales of mica. It is 

 named from Syene, in Upper Egypt, where the an- 

 cients quarried it in blocks of great magnitude. 



Werner having remarked, that this rock was associa- 



MINERALOGY. 





ago, as mentioned in our account of the Hebrides, ob- G*n ? nni 

 served this columnar structure in the granite of Mull ; S ~-Y~"- 1 

 and since that time Humboldt has described it as oc- 

 curring in the granite of Caraccas, as it does in the 

 granite rocks of Carlsbad. 



5. Stratified structure, or Stratification. Granite is Stratified 

 sometimes disposed in great beds in gneiss and other structure, 

 rocks, and occasionally these beds appear divided into or * tratlfi - 

 strata. In other instances, in granite mountains, we catlon ' 

 observe, besides the tabular, globular, and other struc- 

 tures, also the stratified ; but this latter is, in general, 



less perfect than what is observed in gneiss, and other 

 similar rocks. 



6. Beds in Granite. Granite does not afford so many Beds in 

 different beds and veinsas occur in gneiss, mica-slate, and granite. 

 other similar rocks. In Scotland, it sometimes contains 



beds of quartz and of felspar. In Switzerland, beds of 



ted with the porphyries of Saxony, arranged it along quartz in granite have large drusy cavities, the walls of 

 with them, separated it from granite, and described it as which a 



Accidental 



mixed 



parts. 



Porphyri- 



Globular. 



Tabular. 



Columnar. 



a distinct species. On the Wernerian view, therefore, 

 it is distinguished from granite by its hornblende and its 

 situation. Our observations in Scotland oblige us to 

 consider sienite as a variety of granite, as it occurs in 

 the same beds with that rock, and exhibits every varie- 

 ty of geognostical position hitherto observed in granite. 



Accidental mixed parts. Besides felspar, quartz, and 

 mica, the essential constituent parts of granite, and those 

 minerals that take the place of the mica, viz. hornblende 

 and chlorite, others sometimes occur in it. These, how. 

 ever.are to be viewed as accidental. Of these schorl is the 

 most frequent, and the next are garnet and tinstone. 

 The following also occasionally occur either imbedded 

 in the rock, or in veins_ that traverse it, viz. rock-crystal, 

 adularia, chlorite, pinite, actynolite, common opal, to- 

 paz, corundum, fluor-spar, beryl, diallage, epidote, apa- 

 tite, magnetical iron ore, and iron pyrites. 



Granite, besides the granular, exhibit various other 

 kinds of structure, such as porphyritic, globular, tabular, 

 columnar, and stratified. 



1. Porphyritic. When large crystals of felspar occur 

 imbedded in a basis of smaller granular granite, the 

 porphyritic variety is formed. These imbedded crys- 

 tals are sometimes upwards of a foot in magnitude, as 

 is the case with some granites in Saxony, and in other 

 countries. 



2. Globular. Some granites are disposed in round, 

 ish balls or concretions, which are from a foot to seve- 

 ral fathoms in diameter. These balls are sometimes 

 composed of curved lamellar concretions, which always 

 include a harder central mass or nucleus. The spaces 

 between the concretions are filled with granite of a soft- 

 er nature, which decays readily, and thus leaves 

 the harder central masses heaped on each other, or 

 strewed about. Such heaps, or tumuli, have been erro- 

 neously described as rolled masses brought from a dis- 

 tance to their present situation, by the agency of cur- 

 rents that formerly swept the surface of the earth. Ex- 

 amples of this kind of structure occur hi the island of 

 Arran, Bohemia, the Hartz, the Fichtelgebirge, and in 

 Other countries. 



3. Tabular. Some granites, when they are traversed 

 by parallel seams, appear divided into tables. These 

 tables vary in extent from a few inches to several fa- 

 thoms. They appear in some cases to be mere varie- 

 ties of the stratified structure. 



4. Columnar. When the seams are arranged in di- 

 rections parallel to several planes, the granite is divided 

 into columnar masses, which resemble the columnar 

 structure of trap and porphyry rocks. We many years 



which are lined <vith magnificent crystals, and groups of 

 rock-crystal. At Zinnwald, in Bohemia, the tin is work- 

 ed in a quartz bed, situated in the middle of the granite. 

 Beds of limestone are also met with in granite moun- 

 tains, as in the Pyrenees ; and some of them of great ex. 

 tent, having been traced by that excellent observer, 

 Charpenticr the younger, for four leagues, and with a 

 thickness of ninety feet. We need not speak of the 

 beds of gneiss, mica-slate, clay-slate, porphyry, trap, 

 &c. upon which it often rests, and with which it fre- 

 quently alternates. 



7. Metals in Granite On a general view this rock Metal* in 

 contains fewer and less extensive metalliferous veins granite, 

 and beds than the slaty rocks of the primitive class. 



Tin, of all the metals, is that which is most peculiar 

 to granite. Tin- stone occurs in the granite of Corn, 

 wall, Saxony, Limoges, and in these countries is gene* 

 rally associated with walfram. 



Iron is frequent in granite. The mines of Traver. 

 sella, in Piedmont, are situated in a granite which is 

 subordinate to mica-slate. The mines of brown iron 

 ore at Taurynia, and of Fillolo in the eastern Pyren- 

 nees, are in granite. Iron pyrites is frequently found 

 disseminated through granite ; and galena, or lead- 

 glance, graphite, molybdena, bismuth, gold, silver, 

 copper, zinc, manganese, cobalt, are among the metals 

 sometimes met with in this rock. 



8. Formations of Granite. Granite occurs in masses, Formation! 

 often many miles in extent, surrounded by gneiss, mi- of granue. 

 ca-slate, and clay-slate, and so connected with these 



rocks, that the whole may be considered as the result 

 of one grand process of crystallization ; that is, the gra- 

 nite is of cotemporaneous formation with the gneiss, 

 as the gneiss is with the superimposed mica-slate ; and 

 the mica-slate, again, with the clay-slate which rests 

 upon it. In other instances, the granite alternates in 

 beds, often of enormous magnitude, with gneiss, mica, 

 slate, clay-elate, and other primitive rocks, or it tra- 

 verses these in the form of veins. 



9. Decomposition of Granite. Some granites resist, Decompo. 

 for ages, the destroying effects of the weather ; while sitiun of 

 others are resolved into sand or clay in a compara- g tan " e> 

 lively short period. The obelisk which is at present 



in the place of Saint Jean de Latran, at Rome, and 

 which was quarried at Syena, under the reign of Zetus, 

 King of Thebes, thirteen hundred years before the 

 Christian era ; and that which is in the place of Saint 

 Pierre, also at Rome, and which a son of Sesostris con- 

 secrated to the sun, have resisted the effects of the wea- 

 ther for three thousand years. 



C)u the other hand, there are granites, as those in 



