366 Morgan and Tallmon — Occurrence of Bitumen. 



it by a tarry material which, wherever it occurs, is invariably 

 in contact with the shell. The main deposit of this tar is 

 assumed to be on the lower side of the egg as it lay buried. 

 (PL XIX, fig. 1, t.) The contacts between this deposit and the 

 colemanite above it are smooth, gently rounding surfaces, 

 without inclusions of either substance in the other. The 

 second largest deposit of tar is almost diametrically opposite 

 the one just mentioned, on what seems to have been the upper 

 side of the eggshell. While the character of the tar is iden- 

 tical, the nature of the contact is entirely different. A little 

 mass of tar of irregular shape is almost completely surrounded 

 by the colemanite and hangs down from above like a pendant. 

 (PI. XIX, fig. 1, t'.) Other slight deposits of tar in various parts 

 of the egg present only similar phenomena. 'No inclusions of 

 the colemanite in the tar could be anywhere detected. 



The surfaces bounding the cavity in the colemanite in which 

 the tar is included are smooth and roughly spherical. Had 

 the colemanite crystallized freely, we should expect the bound- 

 ing surfaces to be right-lined and planar, conforming to the 

 faces of the crystals, and not spherical as in the present 

 instance. The shape of any deposit of material subsequently 

 laid down must of necessity correspond to those surfaces on 

 which it is deposited. Considering the inclusion of the tar in 

 the mineral, the shape of the included mass and the nature of 

 the contacts, there seems to be no doubt that the entrance of 

 the colemanite took place after the tar had already accumulated 

 inside the shell. Hence it was that the colemanite crystallized 

 about the tar, the nature of the surfaces of contacts being 

 determined by the tar and not by the crystallizing mineral. 



Besides the deposits of tar inside the shell, there is a thin 

 film of the same material between the shell and the matrix, 

 adhering to both. On the inner surface of the matrix, a number 

 of small black prominences are plainly in evidence (PL XIX, 

 fig. 3), which, under the lens, seem to be minute deposits of 

 the same tarry material. They readily disappear when the 

 rock is washed off with chloroform, and, after thorough extrac- 

 tion, in place of the prominences little globular cavities or 

 sacks are found. (PL XIX, figs. 3 and 4.) The diameters of 

 these openings increase after penetrating beneath the surface 

 of the matrix. Microscopically examined, the limestone about 

 these cavities appears identical in character with that removed 

 from their vicinity. The cavities are found to correspond 

 with visible canals in the eggshell. The number of pits in a 

 given piece of matrix corresponds with the number of visible 

 passages in that part of the shell from which the matrix was 

 removed, the relative distribution of each being about the 

 same. A minute crack in the shell is also filled with tar and 



