632 ALLUVIA. 



of these depends on the proportion of this sub- 

 stance in the compound. 



SECOND DIVISION. 



Solid. 



FIRST SUBDIVISION. 



Simple. 



A. Compacted sand of quartz, or recent sand- 

 stones : sometimes found in river alluvia ; tender. 



B. Compacted shell sand : recent oolite. 



This occurs abundantly on the shores of the 

 Bahama Islands ; also in similar situations else- 

 where. The grains are often perfectly rounded, 

 and it serves to explain the origin of the older 

 limestones of this nature. 



C. Compact limestone, deposited from the waters 

 of existing rivers or lakes in large masses. 



The well-known travertino of Rome belongs 

 to this variety. The stalagmitic rocks, such as 

 that of Gibraltar, may also be arranged under 

 this head. 



