EVOLUTION OF LIMESTONE AND DOLOMITE 329 



than in limestone, although no quantitative study of this problem 

 seems to have been made. It has been observed in various places 

 that the limestone beds intercalated between dolomite are full 

 of fossils, while the dolomite is barren. Illustrations of this are 

 found in the Galena limestone of Wisconsin. The paucity of fossils 

 in dolomites has been variously interpreted, some holding that 

 fossils were never present in the formation because the conditions 

 at the time of deposition were unfavorable to life, while others 

 claim that they were originally present but have been destroyed 

 by metamorphism. In connection with the first interpretation, 

 it is interesting to note that magnesium-bearing solutions have 

 actually been found to be unfavorable to life processes. It may be 

 the correct interpretation in the case of certain dense, fine-grained 

 dolomites. On the other hand, porous, cavernous dolomites like 

 the Lower Magnesian of the Upper Mississippi valley, which is 

 nearly devoid of fossils, evidently has undergone considerable 

 recrystallization which may have destroyed the fossil record. 

 No final conclusion seems to follow from the apparent fossil bar- 

 renness of dolomites with reference to the origin of dolomite. 



Association of certain dolomites with gypsum and salt deposits. — 

 Of certain dolomites which are found with deposits of salt, gypsum, 

 and red beds and other indications of aridity and concentrated 

 seas, it has been suggested that they are direct chemical precipi- 

 tates, since conditions of deposition were apparently unfavorable 

 to life. This view is open to the criticism that no case is known 

 where this process is in operation at the present time. However, 

 regardless of what the specific processes of dolomite building were, 

 it seems more likely that these deposits were primarily dolomite 

 because gypsum tends to decompose dolomite into calcium car- 

 bonate and magnesium sulphate. Therefore, if any alteration of 

 the deposit had taken place since deposition, it would seem more 

 likely to result in the decomposition of dolomite than in dolomiti- 

 zation. 



Chemical precipitation of dolomite in the sea. — Crystals of dolo- 

 mite grow in vugs and cavernous openings of coral reefs in the 

 Pacific Ocean, according to E. W. Skeats. 1 



1 E. W. Skeats, Bull. Mus. Comp. Zool., XLII (1903), 53-126. 



