No. 104.] 211 



liornbUnde to deal with as simple minerals of definite constitution. 

 The quartz or silica is in a crystalline condition. The felspar, a 

 crystalliiK) mineral, is composed of a largo proportion of silica with 

 alumina and a small proportion of potash, and often a small amount 

 of soda and lime, with a trace of iron sometimes amounting to more 

 than one per cent. The mica, also crystalline, is composed of silica 

 with a larger proportion of alumina than in felspar, and a lesser per- 

 centage of potash or other alkali, with from three to six per cent of 

 iron. The hornblende is likewise crystalline, and composed of a large 

 proportion of silica with magnesia and lime and sometimes alumina, 

 containing also a variable amount of iron, which sometimes reaches 

 to fifteen or even twenty per cent. 



We have therefore no new mineral substance introduced into the 

 compound. The alumina, which was in mechanical mixture with the 

 silica in the original stone, has combined chemically with a portion of 

 that mineral, including also some potash, soda or lime, and thus pro- 

 duced the felspar and mica. Other portions of the silica, and some- 

 times of alumina, have combined with the magnesia, lime and iron, 

 to produce hornblende. All these materials have existed in their 

 normal condition in the mechanical or sedimentary deposits, and have 

 taken their present form through chemical action during subsequent 

 metamorphism. These crystalline aggregates may be coarse or fine, 

 and the different minerals be present in very variable proportions, or 

 even one or two of them absent from the compound. The prevailing 

 compounds are of quartz, felspar and mica; or quartz, felspar and 

 hornblende. 



The aggregates may likewise be of very different colors, the quartz 

 being usually translucent, the felspar varying from white to reddish 

 brown; the hornblende, of a dark green or black color, while the 

 mica may be of any shade from silvery white to a dark brown or black. 

 The predominance of these, or of any one or two of them, usually gives 

 their hue to the mass. The granites or sienites, in which hornblende 

 predominates, are generally of a dark color; and those where quartz 

 and felspar predominate constitute the lighter-colored granites. 



As a general rule, the granites are more reliable as a durable building 

 material than any other class of stone, and yet some varieties of them 

 are rapidly decomposed by the action of the atmosphere. In these 

 granites where felspar greatly predominates, or where this mineral 

 occurs in large crystalline masses, there is danger of decomposition. 

 The action of the weather upon the alkaline constituents of the min- 

 eral is the primary cause of the destruction ; but this change goes on 

 slowly, and, in the walls of a building, would scarcely affect the ap- 

 pearance of the surface in half a century. The presence of finely dis- 

 seminated iron pyrites is often a cause of destruction in the gneissoid 

 and granitic rocks. 



Some of the fine-grained felspathic granites with mica are subject to 

 a slow decomposition or disintegration of the surface, by which thin 

 films are exfoliated. Such examples can be seen in some of the older 

 granite buildings of the country. Fewer causes of decay are inherent 

 in the ordinary granites than in any other stone used in our build- 

 ings ; and with proper care in selection, a granite structure is com- 

 paratively indestructible from the usual action of the elements. 



