BUn.DING AND ORNAMENTAL STONES. 371 



nary situations, ami tlio case witli wliich it can be worked reii<lers it 

 prefGrablo to the liiuestoues in tlio imiucdiato vicinity. Tlie method of 

 quarrying is to bore holes with a comiuon auger and then blast by 

 means of powder. The blocks are then trimmed to the proper size and 

 shape by means of common wood-saws and hatchets or axes. 



i).— LIMESTONES AND DOLOMITES. 



(1) CHEMICAL COMPOSITION AND ORIGIN. 



Pure limestone consists entirely of calcium carbonate. In point of 

 fact, however, none of our limestones are chemically pure, but all con- 

 tain more or less foreign materials, such as magnesia, oxides of iron, 

 silica, clay, bituminous nuitter, mica, talc, and other minerals. 



In pomi)osition, texture, and general appearance, limestones vary al- 

 most indefiuitely. They may be hard, compact, line-grained rocks of 

 almost flint-like texture, or, again, coarsely porous, oolitic, or crystal- 

 line, the crystals varying in size from too small to be visil)lo to the 

 naked eye to an inch or more in length. 



Pure limestone is white in color, but water blue, gray, green, i^ink, 

 red, and black varieties are common, the colors being de])endent npon 

 various imi)urities, such as the oxides of iron and carbonaceous matter 

 caused by animal and plant remains. The pink and red colors are 

 caused by iron oxides, while the blue, gra,y^ and black varieties owe 

 their hues to the prevailing carbonaceous avitter. The green color of 

 some of the Vermont marbles appears to be due to talc. 



Limestones are regarded by geologists as of either chemical origin 

 eras resulting from the deposition of organic remains, such as shells 

 and corals. Of the first kind are the tufas and travertines 5 of the 

 second, the fossiliferous limestones, such as the cucrinital stones of Ohio 

 and the shell marbles of Tennessee. Either variety may have under- 

 gone the change called metamorphism, and all traces of their origin 

 have been destroyed. 



Limestones occur in stratified beds among rocks of all geological 

 ages, from the Archiean to the most recent. The majority of those used 

 for building and ornamental work belong either to the Cambrian, Silu- 

 rian, Devonian, or Carboniferous ages. 



(2) VARIETIES OF LIMESTONES AND DOLOMITES. 



The following list includes all the principal varieties of limestone 

 popularly recognized, the distinctions being founded upon their struct- 

 ure, chemical composition, and mode of origin : 



Crystalline limestone. Marble. — An entirely crystalline, granular ag- 

 gregate of calcite crystals. The crystals arc usually of quite uniform 

 size in the same marble, but often vary widely in those from different lo- 



