WELL RECORDS 263 



These several records give a fair idea of the lithological character of 

 the sediments for, say, 2,200 feet above the Tamasopa limestone. There 

 appear to be two horizons marked by red or pink-colored shales, the 

 higher of which is seen cropping at the surface near the large concrete 

 reservoir at Ebano, and also in Chapopote Gulch, while the lower red 

 horizon is shown in Pez No. 4 at a depth of 745 feet. This latter horizon 

 was also noted in Laguna No. 17 of the Ebano field at a depth of 721 feet. 



Well No. 5, at Chijol, near Ebano, had a peculiar experience. It was 

 started at a large seepage and struck basalt at 50 feet. It was then 

 moved several feet, but again struck the basalt, and was moved a few 

 more feet to a third location, where the dike was struck at 116 feet, 

 which, after great difficulty, the drill finally penetrated, 21 feet lower, 

 and entered the shales as shown in the drillers' record of No. 5, Chijol. 



These records also reveal the fact that oil can be found at most any 

 depth even in the shales, wherever the drill strikes a fissure, fault, or 

 porous layer in its descent. 



The character of the "oil sand" or reservoir rock was not observed, as 

 none of it had been preserved; but Doctor Hayes says that fragments of 

 limestone thrown out of the great Potrero del Llano well give evidence 

 of much erosion or solution cavities. 



An interesting feature connected with those wells flowing hot salt 

 water is the formation of limestone balls or spheres filled with a regular 

 lace-work of branching cavities very much resembling coralline growth. 

 Doctor Hayes attributes the formation of these spheres to the lessened 

 solubility of the gradually cooling water on its passage up the oil-well 

 vents. Many of these spheres from y 2 inch to 5 inches or more in 

 diameter have been thrown out of the Dos Bocas hot water well, as also 

 from La Pez No. 6. The deposition of the lime probably begins around 

 some nucleus of foreign matter, but Dr. Geo. P. Merrill, of the National 

 Museum, who examined several of these balls, could find but little evi- 

 dence of any distinct nucleus. The following notes and illustrations of 

 these concretions, kindly made for this paper by Doctor Morrill, will 

 prove of much interest: 



"I have examined the concretions from the Mexican oil wells with a good 

 deal of interest, and have to state as follows : 



"The general external form, as readily noted, is either spherical or oval, 

 the surface being at times nearly smooth or again containing numerous per- 

 forations of a millimeter or more in diameter. Cut through the center, they 

 show concentric layers of calcium carbonate, some of which are nearly white 

 in color and very pure, and others so stained with bituminous matter as to be 

 almost black (plate 5, figures 1 to 3). The bituminous matter sometimes 

 impregnates, quite uniformly, various layers of the concretion, but is more 

 commonly contained in elongated cavities, from 1 to 10 or more millimeters in 



