144 THE FORMATION OF FLINTS 



The properties of opal are not yet as completely known 

 as we could wish ; it differs greatly from quartz, par- 

 ticularly in the matter of solubility ; the latter, as we 

 have seen, is to all intents insoluble in rain-water indeed, 







FIG. 39. Venus's Flower "Basket (Euplec- 

 tella aspergillum, Owen), dredged by 

 the Challenger, from a depth of 95 

 fathoms, off the Philippine Islands. 

 About one-third the natural size. 

 After F. E. Schulze. 



it is attacked by few chemical reagents. Opal is certainly 

 more soluble, it is readily dissolved by caustic alkalies, 

 and yields slightly to hydrochloric acid; possibly some of 

 the products of decaying organic matter, or solutions of 

 carbonate of lime may have some solvent effect upon it, 



