60 NEW YORK STATE MUSEUM 



white, and the iron-bearing ingredients (biotite, hornblende and 

 pyroxene) are usually black. By alteration to sericite or kaolin, the 

 feldspar loses its naturally brilliant luster and becomes opaque and 

 earthy. The coloration of some granites arises from infiltration of 

 iron compounds in sufficient amount to overcome the color values 

 of the silicates and impart their own effects. This is well instanced 

 by the yellow Mohegan granite from near Peekskill, the beautiful 

 color of which is traceable to a little limonite that has found its 

 way into the stone by means of the capillary pores. That the 

 color is not due to local alteration of the minerals is very apparent 

 from examination of thin sections which show the only iron-bear- 

 ing silicate (biotite) to be quite fresh in most of the stone and only 

 occasionally is a local deepening of the color observable about that 

 mineral. At the surface the biotite shows some alteration with the 

 production of chlorite, but there is very little iron discharged in 

 the process, altogether too little for the amount of limonite dis- 

 tributed through the body of the rock. Apparently the iron has 

 come from above, probably introduced in solution as a ferrous 

 compound to be subsequently oxidized to limonite. 



SYENITE AND ANORTHOSITE 



Syenite and anorthosite belong to separate rock series but, from 

 a practical standpoint, are much alike. Both consist of feldspar 

 as the essential ingredient, with accessory hornblende, biotite, py- 

 roxene and magnetite. In syenite, the feldspar is an alkali variety, 

 either microcline or orthoclase, or an intergrowth of one of these 

 with albite, known as microperthite. Anorthosite, however, con- 

 sists of a basic plagioclase, usually labradorite, with one or more 

 of the iron-bearing silicates and usually ilmenite in the place of 

 magnetite. 



Their structure is mostly even-granular and compact. As to 

 strength and durability, they are nowise inferior to the granites, 

 if not exceeding them in some elements which make for permanency. 

 In specific gravity they average a little higher than the latter and 

 range from about 2.65 to 2.90, with 2.75 perhaps as a mean value. 

 Their weight is accordingly around 175 pounds to the cubic foot. 



They are not so abundantly distributed as granite, but where 

 they occur they constitute equally large bodies, sometimes forming 

 bosses and bathyliths of great size. 



The color of syenite, and of anorthosite as well, is darker than 

 that of average granites. Green and blue tones are not rare, and 

 the luster from the feldspar is often very brilliant, making the 



