24 NEW YORK STATE MUSEUM 



The appearance of foliation is due to the parallel arrangement of 

 the prismatic and scaly minerals or to the elongation of the quartz 

 and feldspar, accompanied by more or less segregation of the con- 

 stituents in alternating bands. Some rocks evidence the effects 

 of metamorphism by granulation and recrystallization, without the 

 development of any marked foliation. This is true of the light- 

 Colored feldspar-quartz gneisses and of the purer feldspar anortho- 

 sites that are common in the Adirondacks. These rocks, when 

 crushed, present a massive appearance and have a smooth fracture 

 in two or more directions, instead of a single cleavage, like the 

 typical gneisses. . 



CHEMICAL AND PHYSICAL PROPERTIES OF ROCKS WHICH 

 INFLUENCE THEIR COMMERCIAL USES 



Chemical composition. The determination of chemical com- 

 position may afford much information as to the availatjility of rocks 

 for different purposes. Its service in many cases, however, may 

 be said to be rather of negative value, as determining the presence 

 or absence of certain harmful constituents and as a test for the 

 relative decomposition which a rock has undergone under surface 

 weathering. The analysis is of most value when used in connection 

 with the results of microscopic study. 



Limestones are employed in large quantities by chemical and 

 metallurgical establishments, and here an analysis is the first con- 

 sideration. For some uses a magnesian limestone may be preferred ; 

 for others a high calcium variety is wanted; but nearly always the 

 demand requires a limestone with low percentage of impurities 

 in the form of silica, alumina and iron. For Portland cement 

 manufacture the presence of the first two ingredients is rather an 

 advantage, as they take the place of so much clay or shale. For 

 building or engineering work, the analysis plays little part in 

 deciding upon a suitable stone. 



With sandstones the chemical analysis is useful mainly as a guide 

 to the character of the cementing substance, since the sand grains 

 themselves are chiefly quartz. Feldspathic sandstones, which are 

 indicated by the presence of alumina, lime and alkalies, are less 

 durable than the pure quartz kinds, but ordinarily good enough for 

 most construction work. In the case of the igneous rocks, chemical 

 composition has some practical significance, though its place can 

 be supplied often by a careful study of the constituent minerals 

 as usually carried out with thin sections under the microscope. 



