Morgan and Tallmon — Occurrence of Bitumen. 367 



a corresponding tarry ridge is noticeable in the matrix. Since 

 the homogeneity of the matrix offers no evidence to indicate 

 that the little cavities are the remains of passages to the 

 exterior, subsequently filled with more recently deposited 

 material, and since the juxtaposition of cavity in the matrix 

 and canal in the eggshell offers evidence that the cavities 

 were formed from within, there seems to be no reasonable 

 hypothesis other than that the tar at present outside the shell 

 and in the immediate layers of the matrix came from within 

 and w T as forced outward from the center. 



To offer a reasonable explanation of the formation of these 

 pits is not an easy matter. Since they are intimately connected 

 with the canals through the eggshell, and these canals as well 

 as the pits are filled full w T ith tar, it would seem as though 

 some corroding action of the tar might be connected with 

 their formation. Not the slightest acid reaction can be 

 obtained from the tar in any way with litmus or phenol- 

 phthalein, however ; hence the solution of the limestone would 

 hardly seem to be due directly to the tar. 



The ready solubility of calcium carbonate in water contain- 

 ing carbon dioxide naturally suggests itself. However, the 

 formation of carbon dioxide or of organic acids from the 

 original decomposition of the egg can hardly be supposed to 

 have caused the action. It is questionable whether a limestone 

 matrix could form about an egg before its contents had broken 

 down to relatively very stable decomposition products. More- 

 over, if these pits had been present in the matrix when the 

 colemanite came in, they would in all probability have been 

 filled with this mineral. It is necessary to assume either that 

 these pits did not exist when the colemanite came in, or that 

 they were already filled with tar which prevented the mineral 

 from filling them. Since the tar that fills them unquestionably 

 came from inside the shell, the only force considered sufficient to 

 make a viscous tar leave a large cavity to fill a small pit would 

 be due to the crystallizing colemanite. As the crystals formed, 

 the resulting pressure forced the tar through the larger canals 

 in the eggshell. Through the fine-grained matrix it could 

 not go because of its viscosity. 



The fact that the tar filling these pits has an earthy appear- 

 ance, like that produced in the " weathering" of asphalts, may 

 offer a clue as to the formation of the pits. Oxygen dissolved 

 in the waters percolating through the strata in which this 

 specimen was embedded, might account for a local oxidation 

 of the tar as it emerged from the pores of the eggshell, the 

 carbon dioxide formed producing an initial solution of the 

 material of the capsule. As soon as the matrix was appre- 

 ciably dissolved, the tar was forced into the new-formed 



