1864.] T. STERRY HUNT ON LITHOLOGY. 2T 



however be found that the line between the two classes cannot 

 always be distinctly drawn ; inasmuch as rocks containing 

 orthoclase and quartz often include triclinic feldspars such a& 

 albite and oligoclase, and by an admixture of hornblende offer a 

 transition to rocks of the second class. On the other hand, quartz 

 is sometimes found with triclinic feldspars and hornblende in 

 the rocks of the second class. Besides these two feldspathic 

 classes, there is a third small but interesting group, in which an 

 aluminous silicate of high specific gravity, such as garnet, 

 epidote, or zoisite replaces the feldspar wholly or in part. These 

 minerals being basic silicates rich in alumina, the relations of 

 this group are naturally with those of the second class, although 

 varieties of these species are found in rocks which belong to the 

 first class. 



The silico-aluminous crystalline rocks may th us be convenient- 

 ly divided into three families. The first of these includes those 

 rocks in which the aluminous mineral is orthoclase (orthose), 

 from which they may be conveniently designated by the name of 

 the orthosite family. The second includes those in which the alu- 

 minous element is an anorthic or triclinic feldspar, and may be 

 designated as the awor^^osiie family: chemically related to this 

 are those rocks holding as one of their elements nepheline, leucite, 

 orscapolite. The third family includes those rocks which contain 

 an aluminous silicate of high density, as epidote, zoisite, garnet, 

 andalusite, or kyanite, in place of a feldspathide. lolite or dich- 

 roite, which enters into the composition of some orthosite rocks, 

 appears from its atomic volume to be related to the feldspars, and 

 should take its place along-side of anorthite and scapolite as a 

 magnesian feldspathide, while beryl in like manner appears to 

 be a glucinic feldspathide. 



It is worthy of notice, that some feldspars having the crystalliza- 

 tion and density of orthoclase, nevertheless contain large propor- 

 tions of soda. The loxoclase of Breithaupt appears from the 

 analyses|of Smith and Brush to be a true soda-orthoclase (Silliman's 

 Journal [2], xvi, 43) ; while the sanidine or glassy feldspar of 

 many trachytes contains potash and soda in nearly equal propor- 

 tions. The name of potash-albite has been given to some 

 feldspars of this composition ; but the trachytic rocks here- 

 after to be described contain feldspars, which, without being 

 glassy, have the composition of sanidine, together with a cleavage 



