115 



TxiB. CCCLXII. 



SI LEX quartzum arenaceum, 

 Siliceous Sand, 



Class 2. Earths. Order I. Homogeneous. 

 Gen. 4. Silex. Spec. 1. Quartz. 



1 he Sandstone of Fontainbleau is well known to crystal- 

 lize in elegant groups of very neat rhombs, that represent 

 the inverse Carbonate of Lime, containing one third or 

 more of Carbonate of Lime, which appears to guide the for- 

 mation*. I have met with nothing like it as yet in Great 

 Britain. One of my sons, however, some six or eight 

 years since, brought me some flatted Cubes or right-angled 

 Crystals, which he found among sand said to come from 

 the neighbourhood of Ryegate. Some were partly ce- 

 mented by a little Carbonate of Lime, and others were so 

 loose that they could scarcely be handled without breaking. 

 As I know of nothing similar, I represent it here as a ra- 

 rity in Great Britain. I also add a variety of Sand which 

 somewhat resembles the Roe of Fishes, or large-grained 

 Ketton-stone, or Pisolithus. It was brought me by Lady 

 Wilson, who found it at Charlton, and is likewise cemented 

 by some Carbonate of Lime. Something like it is also said 

 to be found at Fontainbleau. 



The darker and irregular specimen is from Oxford, and 



* Exotic Mineralogy, t. 1. See Proposal on the cover. 

 K 2 



