MINERALOGY. 



415 



Sfctr* of 

 oun uin*. 



tome diitricts in Scotland, which are speedily disinte- 

 grated into gravel or sand. But between these two 

 extremes, of extreme durability and rapid decay, there 

 are numerous intermediate degrees. In the same moun- 

 tain, or even in the same hillock, granites of different 

 qualities will sometimes be met with. One portion 

 will be excessively obdurate, and resist long the gnaw- 

 ing efTfrts of the weather ; another variety, imme- 

 diately beside it, will be of a very decomposable na- 

 ture ; and while a third, associated with the two for- 

 mer, will possess an intermediate degree of durabili- 

 ty. Granites vary in their mode of decomposition. 

 Some assume the globular form ; others that of rhom- 

 boidal or irregul.ir masses. These are further disinte- 

 grated, an:l then the constituent parts fall asunder; 

 when a kind of gravel, or sand, depending on the size 

 of the grains, in formed. The felspar in this gravel is 

 further altered, and easily changed into a clay, which 

 is carried into hollows or plain*, and forms beds of 

 clay ; the quartz grains, by attrition, are reduced in 

 size, rout -n, and give rise to beds of sand ; 



and, when mixed with the matter of felspar, to sandy 

 clays. The mica is further broken down, and becomes 

 mixed with tli clays and sands formed from the fel- 

 spar and quarts. 



The soil formed from decomposed granites, is in ge- 

 neral comparatively unproductive. 



10. Shape of Granite Mountain*. In those granite 

 districts in wnich the granite is of a loose texture, and 

 easily acted on by the weather, the hills have a round- 

 ish form, and the lower granite tracts have a waved, or 

 rather a mamillary outline. 



But if very hard and indestructible granite rises 

 through softer and more easily disintegrated, the harder 

 portions appear in the form of peaks, needles, or in 

 deeply deut.ited ridges, or crutsr, and thus give rise 

 to the bristled snd denticulated aspect so peculiar to 

 many granite districts. The valleys, in granite coun- 

 tries, are in general very deep, narrow, and their sides 

 often resemble immense perpendicular walls. 



anite rocks are frequently much traversed by 

 rents or fissures. When these rents widen by the ac- 

 tion of the weather, the mass separates into fragments 

 of greater or leaser magnitude, which remain long 

 piled on each other in a most fantastic manner, often 

 appearing like vast artificial tumuli, or msssts brought 

 together by a flood. The upper parts of the granite 

 mountains in Arran, present very striking appearances 

 of this kind, ami 1 have observed the same in many 

 places of the high granite ridges of the Riesengebiree. 

 Travellers have described similar appearances in the 

 mountains of Switzerland; those of Siberia ; the Hartz; 

 the Bbhmerwald-gebirge, and the Carpathians. 



1 1. Geographical dutribution of Granite. It is one 

 of the most frequent and widely extended rocks. It 

 occurs in almost every mountain group, and there it 

 frequently juts out, forming its central and highest 

 part, having the newer primitive rocks resting on 

 it, or placed beside it. It forms mountains in tK t 

 country, as in the island of Arran, and the centra] part 

 of the Grampians. The same is the case in the Harts, 

 the Riefengcbirjre, the Bobmerwald-gebirge, the Fid*. 



washed away since deposition. Instances of this we Geognosy, 

 have in the Island of Arran, near Carlsbad in Bohe- x "V^ 

 mis, and many other places. 



The following list of localities, shows the known ex- 

 tent of granite in the different quarters of the globe, 

 without, however, any reference to its forming the cen- 

 tre and highest, or the lowest part of mountains or 

 mountain-groups. 



In Europe, it forms the range of Sewoga in Scan- 

 dinavia ; the rocks of Finland ; occurs also in Corn- 

 wall in England, in the Hartz, the Forest of Thurin. 

 gia, Erzgebirge in the Electorate of Saxony, the Fich. 

 telgebirge, Lusatia, the Biesengebirge, the Bohmer- 

 wald-gebirge, the Schwarzwald (Black Forest,) the 

 Alps of Switzerland, and Savoy ; also in the Tyrol, 

 Salzburg, Stiria, Archduchy of Austria, the Carpathian 

 Mountains, Auvergne, Dauphiny, Elsass, and the Py- 

 renean Mountains. 



In Asis, it forms the centre of Caucasus ; occurs at 

 Kolywan, and other places in Siberia; forms a very 

 considerable portion of the Uralian, Altain, and Hi- 

 malya chains of mountain-groups. 



In Africa, it is said to form a principal constituent 

 part of the mountains in Upper Egypt, the Atlas 

 Mountains, and the country about the Cape of Good 

 Hope. 



America. It occurs but in comparatively small quan- 

 tity in the United States ; and in Mexico, owing to that 

 deep and high cover of porphyry, it is found only low 

 down, as at Acapulco. In the Andes of South Ame- 

 rica, it is usually covered with gneiss, mica slate, 

 and trachyte, and in general is not observed higher 

 than 6000 feet : but it abounds in the low mountains 

 and regions of Venezuela, and of Parima, and descends 

 van to the plains and to the level of the sea, as is the 

 case on the sides of the Oroonoco, and the coasts of Peru. 



In North America, it is said to occur in New York, 

 Pennsylvania, and Virginia. In South America, it 

 forms I>e los Mariches, near Caraccas, the whole Cor- 

 dillera of Parima, Sierra Nevada de Merida, Torrito 

 between South Carlos and Valencia, the country be- 

 tween Valencia and Portocabello, and Cape Horn, the 

 southern extremity of America. 



Via. It forms an excellent building and paving u wfc 

 stone, snd has been extensively employed in ornamen- 

 tal architecture. 



Topaz-Rock. 



This rock, which appears to be intimately connect- To 

 ed with granite, lias the following characters. Its con- rotk. 

 stituent minerals are quarts, tourmaline, topaz, and li. 

 thomarge. It is composed of many small masses, 

 which have the appearance of fragments, although they 

 are true granular concretions. Each of these massea 

 is composed of thin layers of quartz, tourmaline, and 

 topaz ; and these layers have different directions in the 

 different masses or concretions. The quartz appears 

 in granular concretions; the topaz is also granular; 

 but is distinguished by its foliated aspect and hardness ; 

 lastly, the tourmaline is in small black needles. Fre- 

 quently hollow spaces occur between these concretions, 

 which are partly filled with crystals of quartz and to- 

 paz, but rarely contains tourmaline. The lithomarge 



the Alp*, particularly those of Savoy; , . 



Bavaria, Bohemia, Franconia, Lusatia, Moravia, occurs amongst these crystals, and has generally an 



Lnper Saxony, Tburingia, Austria, Stiria, and the Ty- ochre yellow, rarely green, and seldom white colour. 



As granite is the basis on which the other pri- It is worthy of remark, that the colour of the crystals 



live rocks sometimes rest, it may also appear in low of topaz, and also its intensity, depends on that of the 



luntainous situations, owing to the newer primitive lithomarge ; as if this substance, or at least its colour* 



rocks either not having been deposited, or hiving been ing matter, wu the same as tbat of the topaz. 



