We passed several small islands; some had but a few stunted trees growing on them; others again had scarcely soil sufficient to nourish a few blades of long wiry grass; while others were barren rocks without verdure of any description, their heads but lately risen from beneath the waves.  I believe that it was at one time supposed that these coral formations rose from immense depths in the ocean, and that those wonderful and persevering polypi worked upwards till they had formed submarine mountains with their honey-combed structures; but it is now ascertained that they cannot exist below at the utmost fifty feet of the surface, and that they establish the foundation of their structures on submarine mountains and table-lands, while they do not work above low-water mark.  How comes it then, it will be asked, that they form islands which rise several feet above the sea?  Although the polypi are the cause of the island being formed, they do not actually form it. They begin by building their nests on some foundation which instinct points out to them.  First they work upwards, so as to form a wall, the perpendicular side of which is exposed to the point whence the strongest winds blow and the heaviest sea comes rolling in.  Then they continue to work along the ground and upwards on the lee side of the wall, sheltered by their original structure from the heavy seas.  They also work at each end of their wall in a curve with the convex side exposed to the sea. Thus, at length, beneath the ocean a huge circular wall of considerable breadth is formed.  Storms now arise, and the waves, dashing against the outer part of the walls, detach huge masses of the coral, six feet square or more, and cast them up on the top of it, where they remain fixed among the rough peaks of coral; and gradually other portions are thrown up, till a mass is formed above high-water mark.  Other bits, ground by the waves into sand, now form a beach, united with shells and various marine productions.  Birds come and settle, and leave seeds which spring up; and trees grow, and attract moisture; and fresh springs are formed, and the spot becomes fit for the abode of man.  Some islands have had a rock, or, perhaps, the plateau of some marine mountain for their commencement, and the polypi have simply enlarged it, and formed a reef around it.  ["The Coral Island," by R.M. Ballantyne, Esquire, Nelson and Sons.] However, this interesting subject has been so often well explained in other works that I will not further enlarge on it, though I could not pass it by in the description of my voyage without some notice.