208 C0RAL8 AND CORAL ISLANDS. 



from the mainland until, opposite Cape Sable, they have so 

 far retreated as to be separated from it by a shallow sheet of 

 water forty miles wide. Farther to the west they project in a 

 more westerly course, with occasional interruptions, as far as 

 the Tortugas [in longitude 83° W.], which form the most 

 western group. They consist either of accumulated dead 

 corals, of coral rocks, or of coral sand, cemented together with 

 more or less compactness. Their form varies, but is usually 

 elongated and narrow, their greatest longitudinal extent fol- 

 lowing the direction of the main range, except in the group of 

 the Pine Islands, where their course is almost at right angles 

 with the main range-^a circumstance which we shall hereafter 

 attempt to explain. 



" Most of these islands are small, the largest of them, such 

 as Key West, and Key Largo, not exceeding ten or fifteen 

 miles in length ; others only two or three, and many scarcely 

 a mile. Their width varies from a quarter to a third or half 

 a mile, the largest barely measuring a mile across ; but what- 

 ever the difference in their size, they all agree in one respect — 

 that their steepest shore is turned toward the Gulf Stream, 

 while their more gradual slope inclines toward the mud flats 

 which they encircle. 



" This is a point which it is important to notice, as it will 

 assist us in the comparison between the Keys and the shore 

 bluffs of the main-land, as well as with the outer reef and reefs 

 of other seas, in all of which we find that the seaward shore is 

 steeper than that turned toward the main-land, or, in case of 

 circular reefs inclosing basins (atolls), than that which borders 

 the lagoon. 



" The reef proper extends parallel to the main range of 

 Keys, for a few miles south or southwest of it, following 

 the same curve, and never receding many miles from it. 



