114 DYNAMICAL GEOLOGY. 



rier-reefs farther and farther south as the necessary con- 

 dition of moderate depth extended. 3. The waves then 

 took, up the work and converted the line of reef into a 

 line of keys, and finally a line of land twelve to fifteen feet 

 high. 4. The shoal waters between the successive lines of 

 keys and the mainland was filled up by coral debris carried 

 inward from the reef and keys, and southward from the 

 previously formed land, and the mainland was thus ex- 

 tended to the keys. 



2. Barrier-reefs without subsidence may be ac- 

 counted for thus : From the manner in which, by this 

 view, the bases of the coral reefs were formed, viz., by 

 sedimentation, there must always have existed a very soft, 

 shallow sea-bottom. Along such a shore-line a, fringing 

 reef could not form., because the chafing waves stir up the 

 mud. But at a distance from shore, where the water is a 

 hundred feet deep, and the waves no longer touch the 

 bottom, a line of reef would form, limited on the one 

 side by the muddiness and on the other by the increasing 

 depth of the water. This would be in form a barrier- 

 reef, but wholly different in significance from those of 

 the Pacific. 



Shell Limestone. 



Lime is constantly carried to the sea by rivers, and yet 

 is the sea-water not saturated. This is because the lime 

 in sea-water is constantly being drafted upon by animals 

 to form their shells and skeletons. These remain after 

 their death, accumulate as lime-deposits, and harden into 

 limestone. We have already spoken of coral limestone, 

 but other animals besides corals form limestones, and 

 some make other kinds of deposits besides lime. Besides 

 corals, the most important are shell-deposits. We shall 

 treat these under two heads, Molluscous Shells and Micro- 

 scopic Shells. And here we would again invite the per- 

 sonal observation of the pupil. 



