160 



QUIGKSILVEU DEPOSITS OF THE PACIFIC SLOPE. 



been taken foi- quartz, but whicli ou separation and analysis proved to be 

 glass/ The inclosing rock contained 67.00 2)er cent, of silica, while the fcl- 

 lowing analysis shows the composition of" the glass : 



Specific gravity '2. 351-2. :563 



Silica, SiO- 70.82 



Alumina, Al-O' 14.01 



Limo, CuO 1.7G 



Water, H-0 0.40 



Alkalis (by (lifferoucc) 7.01 



100. CO 



In this case, therefore, the amorphous material accompanying well crystal- 

 lized components contained nearly 10 per cent, more silica, a large quantity 

 of alkalis, and very little lime.- 



In the basalt of the Rossberg, near Darmstadt, also, Mr. T. Petersen^ 

 found a bottle-green glass inclusion, or segregation, wdiich differed very 

 greatly indeed in its chemical composition from the surrounding mass, as 

 is shown by the following analyses: 



Specific gravity 



SUica.SiO- 



Titiinicacid.TiO' 



Aluinina.ArO' 



Ferric oxi.lc, Fe'O'' 



Ferrous osiilo.FeO 



Mans.innus oxide, MnO . 



Magnesi:!, irpjO 



Lime, CaO 



Soila.Na'O 



Potassa.K'O 



Phosphoric acid, P-0* . . . 



Carhonic acid, CO- 



Wator.H'O 



' Zeitschr. DeiitscU. gcol. Gesell., vol. 17, 1835, p. 413. 



-About tlie time at which this monograph was tr.insraitted, a very elaborate study of the Amtata 

 rocks was published by Mr. J. Francis Williams in the Neues J.ihrbuch fiir Mineral., Y. Beilage-Band, 1887, 

 p. 381. He regards the whole mountain as a single m.issive which is typically developed as trachyte 

 toward the center, but tends sometimes to an andesitic and sometimes to a rhyolitic composition at the 

 edge. The rock is all more or less glassy. A very pure glass from Fosse del Diluvio gave: Sp.gr., 

 2.346; SiO-, 73.57;' CaO, 0.90; MgO, 0.26; Ka-O,3.0:t; K-0, 5.74. An analysisof typical trachyte from 

 the PoggioTraburzoIo gave: Sp. gr., 2.562 ; SiO-, 64.76; CaO, 3.24 ; MgO, 1.74; N.a-0, 2.67 ; K-0, 5.49. 

 As in the case of the Clear Lake andesite and basalt, the glass is more acid than the rock, and the 

 proporlion which the alkalis bear to the earths is much greater in the amorphous material. Here also 

 the gla.ss was prevented from crystallizing by peculiarities of composition, not of the physical condi- 

 tions to which it was subjected. 



= Nenes Jahrluich liir Mineral., 1869, p. 3i'): ibid.. IS7.1. ii.:>87. 



