QUARRY MATERIALS OF NEW YORK 2J 



mineral com'bination among the igneous rocks, provided the in- 

 gredients are not already in weathered condition, is durable enough 

 to serve the purpose of ordinary building construction. There is 

 little choice, so far as mineral composition is concerned, to be made 

 between a granite, a syenite or a gabbro. From the standpoiinits of 

 toughness and resistance to abrasion, which are important qualities 

 for concrete and road materials, the syenites and the more basic 

 rocks are likely to prove superior to the granite. 



The sedimentary rocks may be classified by their mineral content 

 into (a) arenaceous materials represented by sandstones and con- 

 glomerates, (&) argi'llaceous materials or clays and shales, and (c) 

 calcareous materials or limestones. They have a simpler mineral 

 composiiti'on than the igneous types. Sandstones are composed of 

 granular quartz held together by some cementing substance. This 

 may be a secondary deposit of quartz, in which case the rock is called 

 quartzite ; or one of the iron minerals, like limonite or hematite. 

 The argillaceous members consist of very finely divided clayey sub- 

 stances with more or less quartz, calcite, iron ores, etc. They are 

 too soft for constructional stone, but under metamorphism yield 

 slates, schists and gneisses. The limestones consist of the mineral 

 calcite alone, or calcite admixed with dolomite, in the latter case 

 being called magnesian or dolomitic limestones. 



Between the groups of limestones and sandstones as a whole, 

 there is no comparison possible with regard to durable qualities. 

 If the nature of the respective components (calcite and quartz) 

 alone were to be considered, sandstone would be far superior, but 

 there are other factors entering into the question. The size of 

 the constituent particles, the porosity, and the character of the 

 cementing substance, if any, need to be taken into account. 



With sandstones, the character of the cementing substance is 

 more important than any other feature. Some contain very little 

 cement, being held together by the surface adhesion of the particles 

 when brought into close contact. These are apt to be friable and 

 little resistant to physical disintegration. Calcite is a common 

 cement, but rather inferior, since it seems to lose its attachment 

 to the quartz with weathering, and the rock becomes a sugary 

 aggregate. Iron oxide in the form of hematite forms a durable 

 binder and provides an attractive color. 



The highest grade sandstones in respect to hardness, toughness 

 and permanency are those in which the grains are bound together 

 by quartz. Such types are called quartzites and are exemplified by 



