138 ON CORAL REEFS AND ISLANDS. 



from the shore. The conglomerate and tufa are identical through- 

 out, and it may be reasonably inferred, that the whole was once 

 a long point, against which the waves of an ancient ocean acted, 

 unparried by coral reefs. Near the summit of the outer island ? 

 about one hundred feet high, there is a neat circular well, fifteen 

 feet deep and four feet in diameter, worn out of the solid tufa* 

 ceous rock. Its sides are smooth and even, leaving no doubt 

 that it was excavated by the action of water. This natural well 

 is connected below with a horizontal chamber, which opens on 

 the face of the rocky bluff. Another horizontal cavern near by 

 extended several rods into the hill ; but it was too low for explora- 

 tion. The character of the caverns, and the nature of the place 

 — at the head of a deep valley which opens on the sea — tell us 

 of some former age when the ocean washed these shores. The 

 place reminded us of the blow-holes which are still common 

 among these Pacific islands. The horizontal caverns are slowly 

 enlarging, by a kind of exfoliation usual in soft granular sedi- 

 mentary rocks ; the sand above peels off slowly, in consequence 

 of the crystallization of common salt, or some of the saline min- 

 erals which are formed in caverns. This process raises the floor 

 more than the increase in height: so that this enlarging process- 

 actually diminishes the passage. The walls of the well are firm y 

 and are not undergoing this kind of enlargement. 



Are coral islands growing from the depths of the oceans ? 



This question is in many minds, the prominent one connected 

 with coral formations ; and although abundantly answered in the 

 preceding chapters, we give here, in concluding, a more formal 

 reply. This reply is a simple negative ,' and a single fact estab- 

 lishes its truth. The reef-forming coral zoophytes, as has been 

 shown, cannot grow at greater depths than 100 or 120 feet ; and 

 therefore in seas deeper than this, the formation or growth of 

 reefs over the bottom is impossible. 



Coral reefs instead of being a curse to the ocean and the ad- 

 venturous mariner, have afforded the lofty islands a protection 

 against the waves ; they have gathered the debris of the abrading 

 land-streams into shore flats, the seats of many of the villages and 

 cocoanut groves of the islands ; they have produced about these 

 islands large numbers of safe harbors for shipping, with inland 

 navigation for the native craft, besides dotting the waters around 

 with green islets. 



They have also studded the oceans with coral islands, as other 

 lands have disappeared, and established many an excellent haven 

 amid the waves. Like garlands they lie on the waters, but the 

 " everlasting hills" are not more enduring. 



