DOMAIN II. SILICEOUS. 



ceasing to be granular, is sometimes level, some- 

 times conchoidal. When this grit is solid, it 

 strikes fire with steel ; when friable, its hardness 

 can only be judged by the ease with which it 

 scratches steel, and the hardest glass ; but it does 

 not scratch beryl. These characters suffice to 

 distinguish it from dolomite, granular sulphate of 

 barytes, emery, and some horn-stones, the only 

 substances to which it bears some resemblance. 



He then enumerates several varieties: as, 1. 

 the grs lustrt of Haiiy, which betrays its granular 

 texture by its translucency. It forms beds at 

 Montmorency, near Paris, and arrects near Cher- 

 bourg. The very ingenious Gillet discovered that, 

 under a violent blow of the hammer, a regular 

 pyramid or wide cone is often extricated. 2. The 

 white sand-stone found to the south of Paris, and 

 often used for grindstones ; while that of Fontaine- 

 bleau, which is in Very thick horizontal beds, 

 serves to pave the streets of Paris. It is some- 

 times mixed with lime, which makes it effervesce ; 

 but this alteration is more rare than is commonly 

 conceived, and is only observed in the quarries 

 called Belle Croix and Nemours, where are also 

 found the curious crystals in which the quartzose 

 saiid assumes the calcareous form. 3. Ribbon- 

 grit, so called because various colours are dis- 

 played in straight lines or in zigzag : it is com- 



