QUARRY MATERIALS OF NEW YORK 25 



The percentage of silica determines whether the rock is to be classed 

 with the acid (over 65 per cent SiOo), intermediate (55-65 per 

 cent), OT basic (below 55 per cent) groups. In the first group free 

 quartz, which is the most resistant of all minerals to alteration and 

 one of the strongest, is present in quantity. All granites belong 

 to that group. The intermediate group consists mainly of syenites 

 and diorites in which potash and lime-soda feldspars are the main 

 ingredients. These show a higher resistance to physical disintegra- 

 tion than granite, but they are perhaps a little more open to chemical 

 alteration. In this group may be classed also the anorthosites wM'ch 

 are made up of lime feldspar and subordinate pyroxene and which 

 are usually classed wnth the gabbros in the basic division For 

 all practical purposes they can be considered as equivalent to the 

 syenites. The basic group is represented by the gabbros, pyrox- 

 enites, hornblendites and diabases among the more common rocks. 

 They have high percentages of the basic or lime feldspar and of 

 the iron-magnesian minerals, especially pyroxene and hornblende 

 and frequently olivine. They are exceedingly tough, unyielding 

 rocks when fresh and eminently suited for crushed stone, but are 

 too somber in color for most construction purposes. They weather 

 rather rapidly through chemical decomposition with the produ<Aion 

 of hydrated silicates and oxides, such as 'serpentine, talc and 

 limoruite. 



The metamorphic rocks are chemically allied to the igneous or 

 sedimentary types from which they have been derived. 



The presence of sulphides in any building or ornamental stone 

 is undesirable. They are indicated chemically by the percentage 

 of sulphur dioxide in the analysis. Pyrite and marcasite, the 

 common sulphides in rocks, break down readily in the atmosphere 

 to iron oxides which cause unsightly stains upon the surface, though 

 not ordinarily weakening the structure of the rock itself. 



The percentages of carbon dioxide and water in igneous rocks 

 afford valuable criteria as to their relative freshness. Carbon 

 dioxide indicates the presence of calcite which results from the 

 decomposition of feldspar and some of the other silicate minerals. 

 Water in amount above a small percentage is also traceable to 

 secondary products like kaolin, talc and serpentine. 



Mineral composition. According to their relative importance, 

 the rock-forming minerals may be divided into (o) essential 

 ingredients and (b) nonessential or accessory ingredients. The 

 former constitutes the bulk of rock masses, commonly all but a 



