342 EDWARD STEIDTMANN 



are high in calcium and low in magnesium, whereas the sea derives 

 its materials largely from the lands and not from the ocean bottom. 

 Sea water is low in calcium and high in magnesium, and is therefore 

 a more favorable as well as a more universal medium for effecting 

 the dolomitization of the ocean bottom than is underground 

 water for the dolomitization of the rocks through which it circu- 

 lates. The direct evidence for the dolomitization of limestones 

 by underground waters which has been presented shows only 

 local dolomitization along fissures and other openings, principally 

 in the belt of weathering or in localities permeated by waters of 

 unique chemical compositions, such as hot magnesian springs. 

 The occurrence of dolomites of vast thickness and extent and the 

 interstratification of limestones and dolomites cannot therefore 

 find a ready explanation in the mutative agency of underground 

 waters. Excepting for certain local occurrences, dolomite forma- 

 tions seem to have developed in the sea, rather than by the meta- 

 morphism of limestones after their emergence from the sea. The 

 evolution oi dolomite and limestones through a decline in the 

 deposition of dolomite with time, therefore, seems to have a high 

 degree of probability. Consequently, the gradual alternation from 

 the predominance of dolomite in the ancient sediments to the 

 dominance of limestones of a high ratio of calcium to magnesium 

 in more recent times has probably been due to gradual changes 

 in the condition of deposition in the sea. 



WHAT FACTORS CONTROLLING THE DEPOSITION OF CAL- 

 CAREOUS MARINE DEPOSITS HAVE UNDERGONE A 

 GRADUAL EVOLUTION RESULTING IN THE EVOLU- 

 TION OF THE LIMESTONES AND DOLOMITES? 



Four factors directly control the composition of the materials 

 precipitated on the ocean floor: namely, pressure, temperature, 

 life processes, and the chemical composition of the sea. Which of 

 these factors has undergone a progressive change, reflected in the 

 evolution of limestone and dolomites ? It is almost needless to 

 consider whether the pressures on the ocean floor have undergone 

 a progressive evolutionary change within geologic time. All the 

 water-deposited sediments found on the continents appear to have 



