GEOLOGY. 233 



filled up with a crystallizalioii of calcareous spar. The form 

 of the crystals cannot be made out, on account of their 

 extreme tenuity. The color of this limestone, as well as of 

 the loose blocks found upon it, is a light gi-eyish-yellow. 

 This stratum rests upon another calcareous bed, whicli differs 

 from the preceding, in the total absence of organic remains, 

 and in its color, which is of a light blue. Its structure is 

 more compact ; so is its fracture. Its horizontal stratifica- 

 tion is distinct, but, the stratum being thicker, it is more sus- 

 ceptible of being used in building. It produces, in fact, an 

 excellent stone, which admits of being hewn, and which is 

 the chief material used in the construction of the fort. This 

 bed IS from fifteen to twenty feet thick. When examined 

 with the microscope, the rock presents very general signs of 

 crystallization, its texture becomes sub-saccharoidal, and 

 veins of calcspar, of an inconsiderable thickness, traverse it 

 in every direction. There are, also, cavities in which crys- 

 tals of carbonate of lime are distinctly seen. Independent 

 of the building-stone which it yields, this bed is likewise 

 valuable as producing the best lime of any found in the 

 vicinity. Immediately under this bed of limestone, in paral- 

 lel stratification, we observed the sandstone, which consti- 

 tutes the principal mass of the bluff, being about sixty feet in 

 thickness. It is a very friable stone, and, in some cases, the 

 grains of which it is formed are so loosely united, that it 

 appears almost hke sand. Every fragment, if examined witli 

 care, seems to be a regular crystal, and we incline much to 

 the opinion that this sandstone must have been formed by a 

 chemical precipitation, and not by a mere mechanical deposi- 

 tion. The process of its formation may have been a very 

 rapid one, such as is obtained in the manufacture of fine salt; 

 and to this may be attributed the circumstance of its 

 loose texture. The grain is very fine. Its color is white, 



