34 



OUTLINES OF GEOLOGY. 



grains of quartz, felspar, hornblende, &c, has been observed 

 to become so firmly aggregated in less than twelve years, by 

 a calcareous cement, that it may be used for mill-stones. 



2. A similar rock forms on the coast of Ceylon, between 

 Colombo and Negombo. 



3. A coarse calcareous sandstone, on the windward side 

 of the island of Guadaloupe. 



In this formation a human skeleton was found imbedded, 

 and was urged as proof that the date assigned in Scripture 

 for the creation of man was erroneous. More accurate ex- 

 amination has shown the rock to belong to the order we are 

 treating of, and to be long posterior in origin even to dilu- 

 vium. 



4. The delta of the Rhone has been almost wholly con- 

 verted into a solid rock by a calcareous cement. 



74. Modern silicious formations are extremely rare, and 

 are principally remarkable in their connection with the more 

 ancient silicious rocks, of some of which they serve to explain 

 the origin. This, as far as respects the modern formations, 

 is from solution in water. The best instances of this sort 

 are : — 



1. Cylindroidal concretions of silica, found in caverns of 

 the Isles of France and Bourbon ; 



2. Silicious stalactites, deposited by the vapour of water 

 on the sides of the crater of Lancerota ; 



3. The silicious deposit of the Geysers in Iceland ; 



4. The deposits of certain warm mineral springs, such as 

 those of Mont d'Or in France, and Poorgotha in India. 



Allophane, a mixture of the hydrates of silica and alumina, 

 is ascribed to a similar origin. 



75. The salts which are at present forming in alluvial 

 formations are, natron and borax, which are deposited by 

 certain lakes ; nitre and nitrate of lime ; alum, and the sul- 

 phates of soda and magnesia. With them we may class 



