3bo KOCKS. 



Shell marble contains scattered fossils, and may be of dif. 

 ferent colors. It is abundant through the .United States. 

 Crinoidal or encrinital marble differs only in the fossils being 

 mostly remains of encrinites, resembling thin disks. Large 

 quarries are opened in Onondaga and Madison counties, N. 

 Y., and the polished slabs are much used. Madreporit 

 marble consists largely of corals, and the surface consists of 

 delicate stars : it is the pietra stellaria of the Italians. It is 

 common in some of the states on the Ohio. Fire marble, or 

 lumachelle, is a dark brown shell marble, having brilliant 

 fire or chatoyant reflections from within. 



Breccia marbles and pudding stone marbles are the pol- 

 ished calcareous breccia or pudding stone, alluded to on 

 page 346. 



Stalagmites and stalactites (page 116) are frequently pol- 

 ished, and the variety of banded shades is often highly beautiful. 

 The Gibraltar stone, so well known, is of this kind. It comes 

 from a cavern in the Gibraltar rock, where it was deposited 

 from dripping water. It is made into inkstands, letter-holders, 

 and various small articles. 



Wood is often petrified by carbonate of lime, and occasion- 

 ally whole trunks are changed to stone. The specimens 

 show well the grain of the wood, and some are quite hand- 

 some when polished. % 



Marble is sawn by means of a thin iron plate and sand, 

 either by hand or machinery. In polishing, the slabs are 

 first worn down by the sharpest sand, either by rubbing two 

 slabs together or by means of a plate of iron. Finer sand is 

 afterwards used, and then a still finer. Next emery is ap- 

 plied of increasing fineness by means of a plate of lead ; and 

 finally the last polish is given with tin-putty, rubbed on with 

 coarse linen cloths or baggings, wedged tight into an iron 

 planing tool. More or less water is used throughout the 

 process. 



Quicklime. Limestone when burnt produces quicklime, 

 owing to the expulsion of the carbonic acid by the heat. 

 The purest limestone affords the purest lime, (what is called 

 fat lime.) But some impurities are no detriment to it 

 for making mortar, unless they are in excess. Hydraulic 

 lime, which is so called because it will set under water, is 

 made from limestone containing some clay, silica, and often 

 magnesia. The French varieties contain 2 or 3 per cent, 

 of magnesia, and 10 to 20 of silica and alumina or clay. The 



