BUILDING AND ORNAMENTAL STONES. 429 



with black lines, which sometimes assume the most beautiful fern-like or 

 dendritic forms imaginable. 



The rock is intensely hard, tough, and without definite rift. It can 

 therefore be worked only at great cost, and is not regularly quarried. 

 It has been used only locally for rough purposes, as for curbing, steps, 

 and sills. An analysis of this rock is given in the tables. 



G. THE LIPAEITES. 

 (1) ADAPTABILITY FOR CONSTRUCTIVE PURPOSES. 



Tertiary and post-Tertiary rocks of any kind are at present very little 

 used for constructive purposes in the United States, owing, in the case 

 of fragmental rocks, to their state of imperfect consolidation and conse- 

 quent feeble tenacity, and in the case of eruptives to their almost entire 

 absence in those portions of the country that have become permanently 

 settled and where as a consequence there has arisen a demand for a more 

 durable building material than wood. Of the eruptive rocks of this 

 class only the liparites, andesites, and basalts have been at all utilized 

 and these to but a small extent. Their textures are, as a rule, such as 

 to fit them only for the rougher kinds of construction, since, with the 

 exception of the glassy varieties, they will not polish, and their rough 

 appearance unfits them for any kind of interior decorative work. 



(2) MINERAL AND CHEMICAL COMPOSITION OF LIPARITE. 



Under the head of liparites are classed those acid eruptive rocks con- 

 sisting chiefly of quartz and sanidin (the glassy variety of orthoclase) 

 which are not older than Tertiary and which may be regarded as the 

 younger equivalents of the granites, quartz porphyries, and felsite 

 pitchstones. 



In texture they vary from co arsely granitoid rocks, entirely crystal- 

 line throughout, through all intermediate felsitic stages to clear glassy 

 forms. Structurally they vary from fine, compact, even- grained to 

 coarsely porphyritic, amygdaloidal, and sperulitic forms; well marked 

 fluidal structure is common. The prevailing colors are chalky white 

 to dark gray; more rarely greenish, brownish, yellowish, and reddish 

 varieties occur. 



The average chemical composition of liparite (quartz-trachyte) as 

 given by Zirkel is silica, 76.36; alumina, 11.97 ; iron oxides, 2.01 ; lime, 

 1.09; magnesia, 0.56; potash, 3.70; soda, 4.53; specific gravity, 2.55. 



(3) VARIETIES OF LIPARITES. 



According as they are crystalline throughout, felsitic and porphyritic 

 or entirely glassy, liparites are classified as (1) granitic liparites or neva- 

 dites, (2) rhyolites, and (3) glassy liparites as obsidian, pumice, pearlite, 

 and pitchstone. Of these only the felsitic and porphyritic variety rhyolite 

 is now quarried. 



