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Account of a Gelatinous Quartz or Siliceous Sinter, which 

 forms the basis of varieties of Old Red Sandstone. By M. 



T. GUILLEMIN. 



jflLS this interesting mineral occurs in some of the sandstone of 

 this country, we have drawn up the following account from a 

 memoir of Guillemin, published in the Annales des Mines for 

 1826. 



External Characters.^^Th'is mineral is of a pretty pure white 

 colour, which, in some varieties, passes into greyish or yellowish 

 white ; it has a resinous or semiresinous lustre, and passes into 

 dull ; it presents itself in irregular masses ; its fracture is some- 

 times conchoidal, sometimes subconchoidal or even ; it is scarce- 

 ly translucid on the edges ; when dull, it is opaque ; it scratches 

 glass with difficulty, and is scratched by steel ; it is easily fran- 

 gible ; it adheres to the tongue, and is capable of absorbing a 

 large quantity of water ; its specific gravity varies according to 

 the quantity of liquid which it contains. 



When immersed in distilled water, gaseous bubbles are speed- 

 ily disengaged, which rise after one another ; and, at very short 

 intervals, a whizzing noise is emitted, and from time to time 

 cracks are heard ; a fissure then forms, and gives rise to a new 

 column of bubbles. At the end of twelves hours, there are still 

 bubbles escaping ; after eighteen hours the absorption appear? 

 complete. If boiling water be used, the disengagement is much 

 more rapid, and by means of it bubbles are still made to rise 

 from a fragment that has been immersed in cold water for seve- 

 ral hours, and which appears saturated. "A fragment of about 

 ^ve grammes weight, already containing 11.11 per cent, of wa- 

 ter, according to a trial made at the moment, still absorbed 

 14.36, in all 25.47 per cent, at the temperature of six degrees of 

 the centigrade thermometer. A hundred parts of this sub- 

 stance, therefore, saturated with water, contain 20.30. Another 

 fragment of about 10 grammes, dried before immersion, absorb- 

 ed 24.51 per cent, of water at zero, or about a fourth of its 

 weight, as in the preceding experiment. 



These specimens, left to themselves for two or three hours. 



