264 I. C. WHITE PETROLEUM FIELDS UF NORTHEASTERN MEXICO 



length, arranged with their longer axes radiating from the center toward the 

 circumference like spokes of a wheel (plate 10. figure 3). These, one might 

 infer, were formed by the elimination of the petroleum from the fluid in 

 which the concretion was forming. The perforations on the outer shells of 

 some of the concretions are evidently due to the fact that the cavities had not 

 et become sealed by the further deposition of lime < plate 5, figure 4 1 

 Certain irregular forms like that shown in plate 5. lgure 3. have, however, 

 rhe appearance of having been irregularly corroded since the concretion- 

 forming process ceased, whereby a part of the material has been redissolved 

 and the cavity walls etched and greatly enlarged. These latter forms, it 

 should be stated, are so heavily charged throughout with petroleum as to be 

 almost black from core to circumference. 



"I find in but two cases any trace of a foreign body which would serve as a 

 nucleus, and in these cases the material is a mere pinhead of ferruginous 

 and clayey material. In all others the material is lime carbonate throughout, 

 with the exception of the bitumen, and goes readily into solution. This lack 

 of an appreciable nucleus is not surprising, in the present state of our knowl- 

 edge of concretionary formation. 



"Dr. T. Wayland Vaughan has called my attention to the work of G. Linck 

 and others, since corroborated by himself through observations on the forma- 

 tion of the oolitic limestone on the Bahamas, where the lime is first thrown 

 down chemically by the increasing alkalinity of the water caused by bacterial 

 action. Though apparently thrown down as an amorphous powder, it soon 

 begins to gather into the form of minute, almost microscopic concretions witb- 

 out a trace of nucleal material— indeed, in water in which no foreign mate- 

 rial is present. 



"Under the microscope these Mexican concretions show simply concentric 

 layers of radiating lime carbonate, the layers varying somewhat in degree of 

 crystallization, but the material in all cases, so far as tbe cobalt nitrate test 

 can be relied upon, being calcite and not aragonite. 



"Concerning the conditions under which they are formed you are better 

 informed than am 1. The rate of growth is evidently very rapid ; otherwise 

 the bituminous material would be excluded. It is. however, evident that they 

 are not all formed in one continuous process, as on breaking the material falls 

 away in shell-like forms showing surfaces coated with bitumen, and in one 

 case, as shown in plate G. there is in the center a small concretion of only 

 3 or 4 millimeters in diameter, while toward the smaller end of the oval are 

 five starting points from which crystallization radiated upward for a little 

 more than half the length of the specimen. At that point crystallization 

 seems to have stopped and then begun again at three different points, and 

 continued upward for a maximum distance of a little over 25 millimeters. 

 Then the process seems to have stopped again, and finally concluded with the 

 formation of concentric layers about the whole mass, which was continued 

 until the present size and form were assumed." 



Character of the Oil 



The petroleum found in the region examined has an asphalt base and 

 varies in gravity, Baume, from 10%° to 12° in the Ebano district to 21° 



