386 M. Guillemin on a Gelatinous Quartz or Siliceous Sinter 

 Silica, - 97.70>j^^ 



Alumina, 



97.701 

 2.30/ 



It contains no lime, nor have the oxides of iron or of manga- 

 nese been detected in it. I have in vain searched for alkalies by- 

 means of carbonate of lead. 



Observatioois. — This siliceous substance differs from the 

 quartzes and flints in many of its characters, and especially in 

 the density, which, in these minerals, is about 9..Q5 ; but it has 

 a great resemblance to the siliceous sinter (Quartz concretio^mc 

 ihermogene of Haiiy). Both have the same lustre, the same 

 hardness, the some fracture. The density differs but little ; 

 Klaproth found that of the thermogenous quartz to be 1.807. 

 These two minerals appear to be a siliceous jelly scarcely con- 

 solidated ; they are both equally soluble in potash, and tljey 

 have both the property of retaining water, and the power of ab- 

 sorbing a new dose of it. The difference which exists between 

 these two substances is the manner in which they appear in na- 

 ture. The siliceous sinter or thermogenous quartz is almost al- 

 ways in stalactites or concretions in the neighbourhood of hot 

 springs, particularly those of the Geysers in Iceland. A sub- 

 species occurs in the island of Ischia, upon a decomposed gra- 

 nite, and is considered as a volcanic production *. The position 

 of the gelatinous quartz is different. 



Geog7iostical and geographical positions. — It occurs in the 

 Commune of Tortezais, in the Department of the Allier, and is 

 very abundantly diffused there, sometimes serving as a cement 

 to sandstones, and sometimes occurring in the midst of these 

 sandstones, in masses often of considerable size. On the route 

 from Noyant to Cosne, between Bussiere and Tortezais, one 

 of these masses is seen intersecting the road for a length of 30 

 metres, and recurring on each side in the fields in detached 

 pieces over a great extent. 



It is fissured in various directions, without any appearance of 

 regularity. The surfaces exposed to the air are always more or 

 less altered, and pass into floatstone (nectic quartz) I have not 

 been able to meet with it in the form of concretions. If this 



• Vide Jameson's System of Mineralogy, and Manual of Mineralogy. 



