W. H. Melville — Diaspore Crystals. 475 



Its genesis, when in the ordinary association with corundum, 

 emery, margarite, and other minerals, can have little in common 

 with the present case. It has never been reported as an associate 

 of alunite, nor as a decomposition product of eruptive rocks. 

 The only previously observed occurrence connecting diaspore in 

 any way with eruptive materials was recently mentioned by A. 

 Lacroix,* who identified it as a minor constituent of a single 

 block of gneiss, enclosed in a basaltic tufa of the Auvergne. 

 This block is composed mainly of garnet, quartz, orthoclase 

 and oligoclase, with rutile and diaspore, and is considered by 

 Lacroix as an ejected fragment of a much metamorphosed 

 rock, like many others in association with it. He does not 

 refer to the novelty of this occurrence for diaspore, nor discuss 

 its origin. 



There is but little evidence upon which to formulate a 

 theory as to the origin of the diaspore of Mt. Robinson, but 

 after study of the specimens, it seems to the writer probable 

 that it is here a result of the destruction of alunite At La 

 Tolfa, alum is obtained by the slow roasting of the alunite 

 rock, after which water extracts the soluble sulphate, but there 

 is a residue of insoluble hydrate of alumina. Should this 

 roasting and leaching take place in nature, it seems quite 

 likely that subsequent conditions might lead to the crystalliza- 

 tion of the residue as diaspore, making a porous quartz-diaspore 

 rock. Some further agency would seem to be required to 

 explain the occurrence of diaspore crystals in the cavities. 



Aet. LYI. — Diaspore Crystals; by W. H. Melville. 



The crystals of diaspore, which Mr. Cross submitted to me 

 for examination, present two types of combinations of planes 

 referable to the prismatic or orthorhombic system of axes. 

 One type consists of light brown transparent crystals which 

 are elongated in that direction commonly chosen as the vertical 

 axis, and which exhibit a largely developed brachypinacoid 

 plane invariably much striated. This latter characteristic is 

 true though to a less extent for the narrow prismatic planes. 

 The crystals are doubly terminated, and are implanted in the 

 associated rock upon one set of prismatic edges. A drawing 

 of these crystals would resemble that given in Dana's Mineral- 

 ogy, fifth edition, page 168, iig. 173. The following forms 

 were observed. f 



* "Sur l'existence d'une roche a diaspore dans la Haute-Loire." Bui. Soc. 

 fran. de Min., xiii, p. 1, Jan., 1890. 



f The symbols given are those of Miller and of Naumann as modified by Dana. 



