FAYALITE IN CERTAIN IGNEOUS ROCKS 553 



the fayalite found in various phases of tlie series, rather than a com- 

 plete description of these rocks as a whole. 



The nature of the chemical and mineral composition of the 

 series will be briefly referred to again. In the meantime it may be 

 of interest to show how the true character of the fayalite was deter- 

 mined. Under the microscope it appears in ordinary light as a 

 honey-yellow mineral, and under crossed nicols it has bright polar- 

 izing colors, resembling olivine. The minerals associated with it, 

 however, such as an abundance of quartz, orthoclase, and albite, were 

 not the ones usually found with olivine. The composition of the 

 rock, with its very small amount of magnesia and the alteration 

 product of the mineral wholly to iron oxide, free from serpentine, led 

 to the idea that it might be fayalite. 



A rough separation of the mineral was made by means of crushing 

 the quartz syenite (like III) bearing considerable of the unknown 

 mineral, and passing the finely crushed material through a Retgers 

 solution of nitrate of thallium and nitrate of silver. A considerable 

 amount of the heaviest material of the rock, which contained, besides 

 the fayalite, some feldspar and pyroxene, was analyzed to see if any 

 appreciable amount of magnesia was present, and to see if the theory 

 that the yellow mineral was fayalite was probably correct. 



The material was analyzed by Dr. Victor Lehner, and is seen to 

 have been largely iron and silica, and almost entirely free from 

 magnesia, as follows: 



SiOz ------- ^2 10% ' 



AI2O3 ------ 3.54 



Fe^O, - - - - - - 19.86 



FeO ------ 37.56 



MgO - - - - - - - 0.07 



CaO ------ 0.57 



P2O, ------- none 



MnO ------ trace 



TiOa - - - - - - - none 



Undetermined - - - - - 6.30 



100.00 



After this rough test, a careful separation of the yellow mineral 

 was made. With the Retgers solution the heavy minerals of the 

 crushed rock were separated. It was impossible to separate with 



