136 Notices of Memoirs — Petrology. 



The best-known example is that of the so-called fibrous quartz of 

 South Africa. The object, however, of the present paper is to show- 

 that this substance is not an original form of quartz, but is merely 

 a product of pseudomorphism, in which the fibrous structure of 

 a pre-existing mineral has been retained. 



Dr. Wibel has examined two varieties of this African mineral — 

 the one brown, and the other blue. The brown variety occurs in 

 the form of bands in a highly siliceous brown ironstone. Analysis 

 of the fibrous mineral showed that it contains — silica, 5746 ; ferric 

 oxide, 37-56 ; water, 5-15. Treated with hydrochloric acid, the iron 

 is removed, and a white fibrous siliceous material is obtained. 

 Hence the author concludes that the brown mineral consists of a 

 mixture of white quartz and ferric hydrate in the form of Gothite 

 (FeA-H,0). 



Analysis of the blue variety yielded the following results : — 

 silica, 97-27; ferrous oxide, 1-67; lime, 0-15; soda, not determined; 

 water, 0-76. The author is led to regard this variety as a mixture 

 of white quartz with blue krokydolite. He believes that both 

 vai-ieties are pseudomorphs after asbestiform krokydolite ; the brown 

 being the product of a perfect and slow alteration, the blue that 

 of an imperfect and rapid alteration. 



Since writing his paper, the author has had an opportunity of 

 studying microscopic sections of both varieties of pseudomorphous 

 quartz ; and he states, in an appendix, that these observations entirely 

 agree with the conclusions which he had previously expressed. 



F. W. E. 



PETEOLOGY. 



HI. — Note on the Basalt and Hydrotachylyte op the Eossberg, 

 NEAR Darmstadt. Notiz iiber den Basalt und Hydrotachylyt 

 des Eossberges bei Darmstadt. Von Hern Dr. Th. Petersen. 

 Leonhard u. Geinitz's N. JaJirh. f. Mineralogie, u.s.iv. 1873. 

 Heft iv. pp. 385-390. 



THE following minerals have been detected as constituents of the 

 Eossberg basalt, namely — augite, olivine, nej)heline, titan- 

 iferous magnetite, apatite, a plagioclastic felspar, leucite, mica, 

 melilite, and hauyne or nosean. As products of decomposition, the 

 basalt yields osteolite and certain zeolitic minerals. The osteolite, 

 which has recently been worked commercially, occurs in white 

 veins running through the decomposing rock. A specimen of this 

 osteolite, dried at 100° C, yielded 34-7 per cent, of phosphoric acid 

 (P2 O5), corresponding to 75-7 per cent, of calcium orthophosphate. 



The substance described some time ago by Dr. Petersen under the 

 name of HydrotacJiylyte, is found in certain parts of the Eossberg 

 basalt. A peculiar vitreous variety of tachylyte also occurred as a 

 pellucid bottle-green obsidian-like mass, inclosed in the rock. Dr. 

 Petersen publishes his analyses of both these substances, and com- 

 pares their composition with that of true tachylytes from other 

 localities. 



