DIAGENESIS : DOLOMITIZATION 761 



alteration may on the other hand be one of secondary replacement 

 of calcium by magnesium. Such replacement in the sea had appar- 

 ently taken place in the case of coral rock reported by Dana (12: 

 JP5) from the elevated reefs of. Makatea Island in the Pacific; this 

 rock contained 38.7% of magnesium carbonate, whereas such rock 

 usually contains less than 1%. Similar alterations have been re- 

 ported by others, thus Branner {8:264) found 6.95% of magnesia, 

 equivalent to 14.5% MgCOg, in reef rock of Porta do Mangue, 

 Brazil ; the corals of the reef containing only from 0.2 to 0.99% 

 of MgO. Similarly Skeats (28) reports analyses of modern coral 

 rock from the Pacific with 43.3% of MgCOo. Such alterations 

 have also been reported from Funafuti, the deep boring on which 

 showed 16.4% IMgCOs at a depth of 500 feet, 16% MgCOg at 640 

 feet, with much smaller but variable percentages above and below. 

 A boring at Key West showed the highest percentage of MgO 

 (6.7%) at a depth of 775 feet, the percentage of CaO at that depth 

 being 46.53%. At a depth of 25 feet and 1,400 feet the two minima 

 occurred (0.29% and 0.30% respectively). 



Metasomatic replacement through the agency of ground water 

 is also an active means by which dolomites are produced. In some 

 cases it is less efifective than submarine replacement, principally 

 because sea water carries more magnesium than is found in such 

 underground circulation. Where such magnesia is supplied, how- 

 ever, as in regions of decomposing magnesium-bearing rocks, this 

 ground water replacement may be very efifective. The magnesia 

 is of course obtained from the belt of weathering where it occurs 

 as carbonate in older dolomites, etc., or as silicate in crystalline 

 rocks and minerals (garnet, staurolite, tourmaline, chondrodite, 

 chlorite and the zeolites, etc.). The silicates are subject to carbona- 

 tion (see ante, pp. 35, 178), and the carbonate then passes into solu- 

 tion and is carried downward to the belt of cementation, when, on 

 coming in contact with limestones poor in magnesia, replacement 

 takes place. 



Local dolomitizations also occur, as for example at Aspen, Colo- 

 rado, where hot magnesian spring waters rising through the lime- 

 stone locally alter it to dolomite. These are, however, not dia- 

 genetic, but belong to the division of contact metamorphisms. In 

 general limestones which have sufifered orogenic disturbances are 

 more commonly altered to dolomites than those not so disturbed. 

 Thus (Van Hise-34 : cS'o/) the Tertiary limestones of the Coast 

 Range of California and of the Alps are more strongly magnesian 

 than the undisturbed limestones of the same age. This is due to 

 the fact that disturbed and shattered strata of mountain regions 



