THE BRAIN CORALS. 579 



As there is not extant a published account of these interesting- islands, which are so 

 intimately associated with what we have to say about the marine objects of the semi-tropical 

 waters of North America, Ave feel sure that it will be acceptable to the reader to have a some- 

 what detailed account of them. 



The Dry Tortugas, before the late conflict of 1861-'5, was little known to the average 

 reader. The establishment of a military prison there soon made the name a terror to evil- 

 doers, and a synonym for the dreadful. During the two years preceding the "conflict" 

 it had been our fortune to reside at the Tortugas as United States surgeon. Fort Jeffer- 

 son was then in progress of construction. It is an enormous work, involving many millions 

 of money. During these two years the quiet life and delightful association with other officers 

 of the post, and their families, interested in the same pursuits, rendered it an opportunity of 

 exceptional excellence for the study of marine zoology. 



The visitor to this region in years when the post was garrisoned would take the following 

 course : Usually a stop, coming from the north, was made at Key West, the only important 

 inhabited island then on the reef. From there a sail, usually by night, of sixty miles, brought 

 one off Marquesas Keys and Rebecca Shoals. Daylight reveals in the western horizon a long 

 row of castellated structures, impressing one as fairy castles, now illumined by the rays of the 

 rising sun. The vessel now abruptly changes her course, to enter the peculiar winding 

 channel that is so characteristic of the coral reef — five miles from the fortress. Anon there 

 shoots forth a small cloud from the top of the work, and simultaneously rises the garrison 

 ensign, followed at an interval by the booming sound of the sunrise gun. 



In this delightful climate, even during the winter months, this scene is as enjoyable as it 

 is novel. On all sides is the vast ocean. Not a sign else, save the four green-capped islets, 

 slender white strips on the bine sea, with low green bushes on their surface. These now begin 

 to be distinguished. Seven of those small islands, of sizes varying from a quarter of a mile to 

 two miles in length, form a sort of irregular ring around a deep harbor. The intervening 

 space is occupied by the solid reef that has been built up from the sea-bottom, and lies just 

 under the surface, many miles in extent, the entire group being about circular and some seven 

 miles in diameter. The water on this area varies in depth from one foot to twenty, and it is 

 the abode of great numbers of the shoal-water corals, corallines, and algae. In the centre, or 

 nearly so, of the harbor, an islet of sand, formed like all the others on the solid coral basis of 

 the reef, and about thirteen acres in extent. On this island, entirely covering it, is built Fort 

 Jefferson, the largest structure of the kind in the United States. 



Though these little islands look to us like mere sand-spits that any stout gale might 

 demolish, they are grounded in the most endurable of material. The solid area of extended 

 reef around them, just beneath the surface, is as firm as rock. Just at the edge of these 

 islands, on the windward side, the waves break with great violence — the vast ocean depths 

 are behind. The still waters within offer the safest anchorage, reached through the narrow, 

 winding channels. The nature of coral reefs the world over is to grow in such shape as to 

 inclose lagoons with more or less depth of water, which is usually sufficient to float the largest 

 vessels. Hence the great value attached to coral islands in the great Indian Ocean, where 

 passing vessels seek temporary shelter from storms. 



The harbor within these islands is valuable for the navy in time of war, as otherwise the 

 presence of a great fortification here is useless. 



The important elements in the building up of these coral reefs are the Astrean Corals. 

 These are not circumscribed in growth like many others, but are seemingly indefinite in 

 boundary. Immense ledges are seen cropping out of the mud in shallow water. 



The Brain Corals, so called from their resemblance to the brain, Meandrinas. from the 

 meandering nature of their cells, exhibit a number of beautiful shapes, ranging from the most 

 regular hemispheres to masses of indefinite shape and size. These, with the star corals, the 

 astreas, as we have seen, form important elements in the building up of reefs. 



In the coral regions of the West Indies and the Florida peninsula the islands are called 

 cays, in English keys, a corruption from cayo, Spanish for an islet. The principal cay of 

 Florida, or the onlv considerable one inhabited, was early called Cayo hueso. Bone Cay, or 



