BUILDING AND ORNAMENTAL STONES. 297 



noii-alu urinous varieties occur in gneiss, crystalline limestone, and other 

 metamorpbic rocks. 



The hornblende in such rocks as are used for building purposes can 

 be readily recognized by its dark-green or almost black color and the 

 compactness and tenacity of its crystals which are not easily separable 

 into thin leaves or folia as is black mica, with which it might otherwise 

 be confounded. Hornblende acquires readily a good and lasting polish 

 and as the mineral itself is strong and durable, its presence in a rock 

 is thought to be preferable to that of mica. 



THE PYROXENES. Hardness, 5 to 6. 



Two principal varieties of this mineral are recognized, as with the am- 

 pinholes, (1) the non aluminous, including the light-colored varieties 

 malacolite, sahlite, and diallage, and (2) the aluminous, including the 

 dark variety, augite. 



The lighter-colored non-aluminous varieties, malacolite and sahlite, 

 are common in mica and hornblendic schists, gneiss, and granite, though 

 seldom in sufficient abundance to be noticeable to the naked eye. The 

 foliated variety, diallage, is an essential constituent of the rock gabbro, 

 and is also common in serpentine. The darker-colored aluminous vari- 

 ety, augite, is an essential constituent of diabase and basalt, and also 

 occurs in many syenites, andesites, and other eruptive rocks. 



In such rocks as are used for building purposes the pyroxene can not 

 usually be distinguished by the unaided eye from hornblende. With 

 the exception of the Quincy granites and the New Castle, Del., gneisses, 

 pyroxenes do not occur in any of our granitic rocks now quarried, but 

 in the diabases and basalts the augite is a very important constituent. 

 It is usually a compact and tough yellowish-green or nearly black min- 

 eral, and, like hornblende, readily acquires a good and lasting polish. 

 The pyroxene of the Quincy granite, however, proves an exceptionally 

 brittle variety, and the continual breaking away of little pieces during 

 the process of dressing the stone makes the production of a perfectly 

 smooth surface a matter of great difficulty. 



CA.LCITE. Calc-spar. — Composition : Calcium carbonate, CaC0 3 = carbon dioxide, 

 44 per cent. ; lime, 56 per cent. Hardness, 3. 



This is an original constituent of many rocks, such as limestone, 

 ophiolite, and calcareous shale, and is the essential constituent of most 

 marbles, of stalactites, travertine, and calc-sinter. It also occurs as a 

 secondary constituent resulting from the decomposition of other min- 

 erals, filling wholly, or in part, cavities in rocks of all ages, such as 

 granite, gneiss, syenite, diabase, diorite, liparite, trachyte, andesite, and 

 basalt. 



Calcite when pure is white in color, and soft enough to be cut with 

 a knife. It can be readily distinguished from other miuerals (exceptiug 

 aragonite) by its brisk effervescence when treated with a dilute acid. 



