ALTERATION OF REEF ROCK 445 



duce a varied aspect in the sections where they are exposed. Char- 

 acteristic examples of sheet reefs of this type are found in the up- 

 permost Siluric beds (Manlius) of central and eastern New York 

 and in the mid-Devonic limestones (Transverse group) of western 

 Michigan. 



Loss OF Structure Through Alteration of Reef Limestones. 



In many of the foregoing examples of fossil reefs the organic 

 structure of the limestone has been wholly or almost completely de- 

 stroyed through subsequent alteration of the rock. In the Niagara 

 reefs of Wisconsin the stromatoporoid structure is scarcely recog- 

 nizable, except in a few cases on weathered surfaces. The same is 

 true of the Devonic and Carbonic reefs of Belgium, and especially 

 of the dolomites of the Tyrol. In all these cases dolomitization is 

 one of the chief factors in the destruction of the organic structure, 

 while recrystallization of the undolomitized limestones has had a 

 similar effect. Such changes are not uncommonly observed in mod- 

 ern coral ^mestones, where the hard parts, composed of aragonite, 

 as in reef-forming corals (Madreporaria), gastropoda and some 

 calcareous algae are rendered unrecognizable by recrystallization as 

 calcite, and where dolomitization obliterates the structure of even 

 the more stable calcite organisms. Sections of modern coral lime- 

 stones have shown that three types of mineral structure may be 

 recognized, due to different degrees of alteration of the rock. 

 (Skeats-8i : 705; 82 : 128; Cullis-19 : 404.) 



1. Limestones consisting largely of calcite, but containing also 

 organisms with hard parts of aragonite, while not infrequently sec- 

 ondary aragonite has been deposited in crystallographic continuity 

 with the aragonite of the organism. 



2. Limestones produced from gradual alteration and recrystal- 

 lization of the first type of limestone, thus resulting in a rock in 

 which organic remains and matrix alike consist entirely of calcite. 



3. Limestones produced from either the first or second type by 

 the replacement of some of the calcium carbonate by magnesium 

 carbonate, resulting in the occurrence in the rock of generally idio- 

 morphic crystals of dolomite ; continued replacement will result in 

 the complete change of the limestone to dolomite. When the rock 

 is completely dolomitized, it becomes usually quite structureless, 

 the crystals of dolomite being almost entirely allotriomorphic* 



* See, further, dolomitization in Chapter XIX. 



