ORIGIN OF THE OOLITES 595 



This interference is characteristic of this type of oolites. A large num- 

 ber of slides of calcareous oolites from all parts of the United States were 

 examined, and in not a single instance had the oolites interfered with 

 each other's growth. They were always merely in contact. This is sig- 

 nificant as showing that the oolites were growing in the muddy water, 

 and that their growth was not due to material acquired by rolling about 

 on the bottom. 



5. There is a slight flattening of the oolites in the yellow shale, but it 

 is not noticeable hi the other phases. This flattening was produced dur- 

 ing the consolidation of the shales, but after growth of the oolites had 

 ceased. 



6. The growth of the oolites was stopped by the accumulation of suffi- 

 cient mud above the oolites to prevent any more silica reaching them. 

 This was evidently the factor which produced the uniform size of the 

 oolites, and it points to a uniform rate of deposition. 



7. The beds which contain the fossils were deposited in shallow water 

 and were probably loose, soft, sandy muds. Branson^ has cited the evi- 

 dence and come to the conclusion that the red beds are, in the main, of 

 marine origin, but that the Popo Agie beds show several evidences of 

 subaerial origin and were formed as marginal deposits, partially marine 

 and partially subaerial. 



The writer believes that the Popo Agie beds were practically continu- 

 ously under water, but that it was very shallow water and comparatively 

 near the shore. 



Tliese points, in connection with the points made in regard to the silica, 

 are believed to be favorable to the view that these siliceous oolites were 

 deposited at the same time as the inclosing shale. 



EVIDENCE AGAINST THE REPLACEMENT THEORY 



Since the prevalent view^ is that all siliceous oolites are due to the re- 

 placement of calcareous oolites by silica, it will be of value to note the 

 evidence against this view in this case. The following reasons are be- 

 lieved to be against it : 



1. The oolites occur in shale. The writer has found no record of eitlier 

 siliceous or calcareous oolites occurring in shale. Under tlio above pre- 

 cipitation theory the occurrence of oolites in shale would not bo unlikely, 

 however. Leith and Mead^ have suggested that "a large part of the silica 

 carried in solution ])y rivers is deposited with the muds and clays'' (page 



^ E. P>. Branson : Origin of tlie Red Bods in western Wyoming. Bull. Geol. Soc Am., 

 vol. 2G, 1915, pp. 217-230. 



8 C. K. Leith and W. .7. Mead : Metamorpliic geology, 191G, pp. SI, 102-104. 



