Mokgan-Ta llmon . ] 



A Fossil Egg from Arizona. 



407 



material contains the normal constituents in the normal propor- 

 tions. 



These facts tell us something of the history of the fossil 

 during the period in which it lay buried, and also show the rela- 

 tion of the tar it contains to other bituminous matter. Since 

 the tar is completely soluble in petroleum ether without residue 

 of any kind, while the heated product is largely insoluble in the 

 same menstruum, it is evident that the fossil has never been 

 subjected to a temperature as high as 150° C. The tar as it 

 exists in the egg requires simply this slight elevation of temper- 

 ature to make it indistinguishable from a natural asphalt. 



While the colemanite is often in direct contact with the 

 shell, the bituminous material is always so. A large portion of 

 the tar content is collected in one body on what is assumed to be 

 the lower side of the egg (PI. 49, fig. 1, /')• Other deposits lie 

 close to the shell or protrude further into the colemanite, and 

 look like apophyses projecting or depending from the roof of the 

 cavity (PI. 49, fig. 1, t') . At one point a smooth round mass of 

 the bituminous matter is almost surrounded by colemanite. 



The relation of the bituminous matter to the colemanite 

 shows conclusively that the tar was present in the shell before 

 the mineral accumulated. When the colemanite entered, a pool 

 of tar lay on the floor of the cavity aud smaller quantities were 

 attached to the sides and roof. As the colemanite filled the 

 shell, a pressure, due, possibly, to crystallization, was developed 

 and forced some of the tar out through pores in the shell. A 

 local oxidation or decomposition of the tar may have occurred, 

 producing carbon dioxide, which dissolved the limestone about 

 the ends of the pores and made the cavities or pits of the matrix 

 which were filled with the expressed tar as rapidly as they 

 formed. 



While it is easily shown that the tar preceded the colemanite, 

 we naturally inquire concerning the original source of the tar. 

 The explanation which first suggests itself is that it may be a 

 part of the original contents of the egg. It is also possible 

 that the tar may have come in from outside after the egg was 

 inclosed in the rock. In its present condition, however, it is too 

 viscous to make the possibility of its passage through the com- 



