156 



ORGANIC AGENCIES. 



2. We have here the formation of limestone rocks of various kinds 

 going on before our eyes over immense areas and several thousand feet 

 in thickness, and we learn thus that limestones are of organic origin. 



3. The character of the rocks thus formed is very interesting to 

 the geologist. In some places, as we have already seen, it is a coarse 

 conglomerate, or breccia, composed of fragments of all sizes cemented 

 together ; in some places it is made up entirely of rounded granules of 

 coralline limestone (coral-sand), cemented together, and forming a pe- 

 culiar oolitic rock (wov Ai#os, egg-stone). But the larger portion of 

 the reef ground is a fine compact limestone, made up of comminuted 

 coralline matter (coital mud), cemented together. This fine coral mud 

 is carried by waves and tides into the lagoon, and serves to raise its 

 bottom : it is also carried by currents and distributed widely over the 

 neighboring sea-bottoms. Soundings in coral seas bring up everywhere 

 this final coral mud, showing that compact limestone is now forming 

 over wide areas in coral seas. The reef-rock, as already stated, has been 

 found clinging to the sides of high islands, having been elevated many 

 feet above sea-level ; in other cases atolls have been elevated 250 feet 

 above the sea-level. The structure of the reef -rock has thus been ex- 

 posed to view. In some places it contains imbedded remains of corals 

 and shells, but in other parts it is entirely destitute of these remains. 



Beefs of Florida. 



The reefs of Flor- 

 ///// *^ a Reserve a bri^ 



separate notice, both 

 because they are dif- 

 ferent from those of 

 ill the Pacific, having 



™ been formed under 



different conditions, 



-'"o "~" -,.. and because they are 



<4 much more efficient 

 -i I ^-~^\ )) agents in land-mak- 



S-M I '■''r^'""">- tug, anc ^ illustrate in 



~^M' Ill/Ill \ Q \ a striking manner 



Bahama j } 10W different agen- 

 <ol. cies co-operate for 

 this purpose. The 

 process has been ac- 

 curately observed. 

 s Description of 



Fig. 127.— Map of Florida with its Keys and Reefs, a, Southern Florida. Fig 1 . 127 is 



coast; a\ Keys; a". Living reef: e, Everglades; e', Shoal water; * & ' 



e", Ship-channel; G S 8, Gulf Stream. a DUl]) of Florida, 



