474 W. Cross — Alunite and Diaspore, 



etc. 



The facts which, have been given seem to prove that the 

 crystals in question are pseudomorphs of alunite after itself. 

 An explanation of this anomaly is suggested by the volcanic 

 history of the district. The crystals are almost certainly the 

 result of solfataric action connected with the occurrences al- 

 ready described. If we suppose those alunite crystals to have 

 been replaced by some other mineral, in which the bases were 

 retained, a renewal of solfataric activity would naturally con- 

 vert them into alunite again, but the protected cores of the 

 original mineral could not influence the orientation of the new 

 generation. 



V. Origin of the rocks described. 



In all of its observed occurrences alunite is a secondary 

 product due to the action of sulphurous or similar acids upon 

 highly aluminous rocks, yet there are two very different sites, 

 with different attendant conditions, in which this action takes 

 place. In the one case, the rock belongs to a clay bank 

 and the acid is derived as a rule from the decomposition of 

 marcasite in or adjacent to the clay. The product is a dark 

 dense amorphous mass. In the other case, the agent is the 

 sulphurous exhalation of a solfatara, and the rock acted upon 

 is usually volcanic and rich in alumina and alkali — such a rock 

 as rhyolite or trachyte. The product of this action is com- 

 monly a hard, rough, porous, highly crystalline rock, though a 

 dense amorphous mass is often locally developed. 



The alunite rock which has been described is directly com- 

 parable with the classic solfataric occurrences of Bereghszasz 

 in Hungary, the island of Milo, and La Tolfa, near Rome. 

 The writer has been unable to find mention of any similar 

 occurrence on the American continent, and, judging from the 

 descriptions given by von Richthofen* and vom Rathf of other 

 localities, the rock of Democrat hill is remarkable for its 

 purity, homogeneity and extent. Exactly similar material is 

 not described by the authors named. 



The limits of this article forbid any further comparison of 

 occurrences or discussion of the processes involved in the 

 formation of alunite, which are reserved for the monographic 

 report upon the geology of the district. 



While the alunite may be assumed to have been formed by 

 the same processes which are involved in other cases, the dias- 

 pore of Mt. Robinson has apparently originated under condi- 

 tions very different from those of any other known occurrence. 



* Id " Studien aus den ungarisch-siebenburgischen Trachytgebirgen " Jahrbuch 

 d. k. k. geol. Reichsanstalt, xi, 254-268, Vienna, 1860. 



f Mineralogisch-geognostische Fragmente aus Italien. IV. Das Bergland von 

 Tolfa. Zeitscbrift der deutschen geol. Ges., xviii, 585. Berlio, 1866. 



