• BUILDING AND ORNAMENTAL STONES. 295 



or oligoclase, or freqiiontly microcline. Anortbite and labraciorite are 

 equally important constituents of basic eruptive rocks, sucli as diabase, 

 basalt, and andesite. 



Tlie physical condition of the feldspar iu a building- stone is a matter 

 of the greatest importance. In those rocks which withstand the effect 

 of the weather through long periods of years without change or disin- 

 tegration, the feldspars, if examined with a microscope, will be found 

 hard, compact, and fresh, containing but few cavities or impurities. 

 On the other hand, the feldspars of many rocks, if thus examined, will 

 be found filled with minute cavities and flaws which are often so filled 

 with impurities and products of decomposition as to be quite opaque 

 (Hawes). Such rocks will not for any length of time withstand the 

 weather, since infiltrating waters containing minute quantities of car- 

 bonic and other acids, aided by heat and frost, can not fail to jiroduce 

 the dire result of disintegration. 



The feldspars have also an important influence upon the cutting of a 

 stone. The hardness and toughness of many granites and other crys- 

 talline siliceous rocks are due, not to the hard and brittle qu;irtz, but 

 to the feldspathic constituent, which is quite variable. The soft gran- 

 ites consist of the same constitueuts, but the feldspars are porous and 

 therefore offer less resistance to the cutting tool. The feldspars also 

 possess a distiuct (tleavage, that is, they split or cleave in one or two 

 directions much more readily than in others. It therefore, sometimes 

 happens, especially iu coarse-grained and porphyritic rocks, that it is 

 very difficult to obtain the perfect surface necessary for polishing, since 

 little particles of the feldspars are constantly splitting out, leaving 

 small cavities or ''nicks." 



The color of a rock frequently depends largely nj)on its feldspathic 

 constituent. If the feldspar be clear, transparent, and glassy, the light 

 enters it and is absorbed, giving to the stone a dark color, as is the 

 case with the Qnincy granites and many quartz porphj^ries and dia- 

 bases. If the feldspar is soft and i:)orous, the light is reflected from the 

 surface and the rock appears white. In all the piuk and red granites 

 and gneisses the color is due to the pink and red orthoclase they contain. 

 It sometimes happens that the orthoclase and plagioclase — M^hen both 

 are present in the same rock — are differently colored, the orthoclase 

 being pink or red, while the plagioclase is nearly white. 



THE MICAS. Hardness 2.5 to 3. 



Two kinds of mica occur as prominent constituents of builduig stones, 

 especially the granites and gneisses. 



These are black mica or biotite, and white mica, or muscovite. Both 

 kinds occur in small shining scales which are sometimes nexagonal in 

 outline, though more frequently of quite irregular ibrm. 



Tlie composition of the micas is complex, but the black variety is es- 

 sentially a silicate of iron, alumina, magnesia, and potash, wliile the 



