HILL: GEOLOGY OF JAMAICA. 



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often accompanied by shells of Mollusks, Echini, etc. In greatly altered 

 reef rock the coral heads and marls become consolidated into firm semi- 

 crystalline white limestones, marked by irregularity of texture and 

 numerous minute cavities representing the original interspaces of the 

 coral skeleton, or sometimes representing places from which the coral 

 structure has been dissolved away. These cavities are frequently filled 

 with red clay or sinter. 



This reef rock not only in Jamaica, but in Barbados, Cuba, Panama, 

 Haiti, Guadaloupe, and elsewhere, has an individuality whereby it can 

 usually be distinguished from white limestones of other than reef rock 



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Figure 28. Showing Composition of Old Reef, 18 Mile Post, near Hopewell. 



origin. Even when greatly altered by interstital change, true reef rock 

 shows by the traces of reef coral structure and by its rough, cavernous, 

 crystalline, and altogether unhomogenous testure its original nature. 

 Hand specimens showing no coral structure can be selected from a mass 

 of reef rock, for mud, coral debris, molluscan shells, and even shallow 

 water Foraminifera often fill considerable spaces between the coral heads. 

 Such specimens are exceptional occurrences, however, and do not war- 

 rant the frequent generalization that great masses of limestone, which 

 show no signs of coralline texture, may often be of coral reef origin. It 

 may be considered reasonably certain that a rock mass in which remains 

 of reef corals are not visible is not coral reef rock.^ 



1 " Coral reef rock is of very varying composition ; the coral grows in hummocks 

 separated by more or less narrow spaces which are filled up by coral sand, broken 



