INTERPRETATION OF ENVIRONMENTAL CONDITIONS 215 



famous for their bad odor. In central Wyoming, phosphate rocks of this kind 

 contain so much oily matter that they are being successfully exploited for 

 petroleum. Although such phosphates contain a few fossils such as fish teeth, 

 brachiopods, and larval gastropods, they are invariably devoid of sessile bottom- 

 inhabiting organisms, a fact which suggests that the bottom layer of sea-water 

 lacked the oxygen necessary to support life. 



"The deficiency of oxygen is, therefore, the controlling chemical condition, 

 for it not only determines that the bacterial decay shall be of the anaerobic type, 

 but also prevents animal scavengers from devouring such organic matter as may 

 fall to the sea bottom, for no animal can be active in an oxygen-free medium. 



4s 4: * 



"As was long ago pointed out by Bonnet, under ordinary circumstances all 

 of the products of decay are likely to either remain in solution or escape as gases 

 rather than to be precipitated. Under special conditions, however, most of them 

 remain in solid form and others react with the sediments of the bottom or with 

 materials in solution, in such a way as to form insoluble products. For example, 

 hydrogen sulphide, interacting with the iron compounds, forms the mineral 

 pyrite, which is common in certain types of black shales. In a similar way, 

 phosphoric acid in the presence of ammonia reacts with various substances, and 

 especially lime carbonates, in such a way as to produce phosphatic minerals, of 

 which the commonest is collophanite, said to be hydrous calcium carbophosphate. 

 These changes have been carried out experimentally in the laboratory by several 

 investigators, and the necessary conditions are such as may readily occur on the 

 sea bottom where organic decomposition is in progress. The calcareous shells 

 and fragments lying on the ocean floor thus become phosphatized, and even such 

 organic materials as excretory pellets and pieces of wood are known to have been 

 altered in the same way. Bones, which initially contained about 58 per cent 

 tricalcium phosphate, have their organic matter completely replaced by phos- 

 phatic minerals, thus raising the ratio to 85 per cent or more. In addition, 

 collophanite is precipitated in concentric layers around particles of sand or any 

 solids, forming round or elliptical granules which resemble the oolitic grains in 

 certain limestones. By the enlargement of these coatings, the granules, shells, 

 teeth, and other objects are cemented into hard nodules or even into continuous 

 beds of phosphatic rock. Such nodules have been dredged up from the bottom 

 of all the oceans in moderate depths, and are not uncommon in certain kinds of 

 marine limestones and shales now on land." 



It is shown by Clark^ that apatite and calcium phosphate are soluble 

 in carbonated waters and waters containing humic acid. The phosphate 

 is deposited, however, in the presence of calcium carbonate. The reaction 

 Ca3(P04)2 + 2H2CO3 -> 2CaHP04 + Ca(HC03)2 is reversible, but will only 

 be complete from left to right when all the carbonic acid is neutralized. 



As the phosphates of the Basin Province are very largely in the petroleum- 

 bearing shale, it is probable that the precipitation of the phosphate was 

 due rather to the real absence of much CO2 than the contact of the dissolved 

 phosphate with limestone. This is a decided contributory proof of the 

 stagnant condition of the sea and the action of anaerobic bacteria. 



» Clark, F. W., Data of Geochemistry, U. S. Geological Survey Bull. 616, p. 519, 1916. 



