26 NEW YORK STATE MUSEUM 



few per cents of the whole; and includes all those that have any 

 considerable influence upon the physical properties and fitness of 

 the materials for economic uses. There are a few exceptions to be 

 made with reference especially to the iron oxides and iron sulphides 

 which occur in small amounts, but yet are important, the former 

 as coloring agents and the latter owing to their tendency to de- 

 compose in the atmosphere and cause unsightly stains. 



The various representatives of the igneous rocks are combina- 

 tions of a small number of essential minerals. A list of the more 

 important minerals includes quartz, feldspar, mica, amphibole, py- 

 roxene and olivine. If to these be added nephelite, sodalite and 

 leucite, which occur in certain areally restricted but not altogether 

 rare types, the list of essential ingredients for the igneous class 

 is complete. 



It may be noted that all the minerals named contain silica. Quartz 

 is silica alone, while the others are compounds known as silicates 

 in which silica functions as an acid and combines with some of the 

 basic elements like sodium, potassium, magnesium, calcium, iron 

 and aluminum, to name the more common ones. Several of the 

 minerals, namely, feldspar, mica, pyroxene and amphibole, are not 

 single species, but mineral groups with a number of individual 

 species possessing similar but not identical chemical and mineralog- 

 ical properties. 



The strength and durability of the igneous rocks in ordinary 

 service are conditioned by the nature of the constituent minerals 

 and the manner in which they occur. The harder and more durable 

 ingredients are quartz and feldspar, consequently the rocks that 

 are made up of them in larger part are the most serviceable under 

 equal conditions. Quartz is not subject to chemical decomposition, 

 but feldspar yields slightly to atmospheric agencies and in the 

 course of time may become softened so as to crumble under pres- 

 sure. The iron-bearing silicates which are represented by mica, 

 amphibole, pyroxene and olivine are also subject to change under 

 the weather, with the result that the iron is partly discharged from 

 combination as limonite, and new combinations of silica character- 

 ized by the presence of water in considerable amount are formed. 

 Chlorite, serpentine and talc are common secondary minerals result- 

 ing from their alteration. It may be noted that while such changes 

 have taken place in nature on a great scale, the element of time 

 has been a factor for which no equivalent can be found within 

 the limits of human experience. As a matter of fact, almost any 



