584 F. G. CLAPP CLASSIFICATION OF PETROLEUM AND GAS FIELDS 



accompanied by many of the Texas type of phenomena. The Mexican 

 plugs are arranged in straight lines in a manner similar to those of the 

 saline domes. An interesting fact mentioned by Hill is that hot water 

 has been encountered in several of the saline domes. 



5. That the domes are situated at points of weakness overlying the 

 intersection of fault-lines, and that heated waters, saturated with mineral 

 in solution, have risen along these points of weakness under intense pres- 

 sure, carrying with them the sodium chloride, sodium sulphate, etcetera, 

 which were deposited near the surface by a relief of pressure and tem- 

 perature. The deposition of these minerals was naturally attended by 

 crystallization, the power of which is supposed to be so great that the 

 entire overlying sediments were pushed upward and outward, forming 

 the domes. This theory seems to have originally been formulated by Hill 

 and is the one now most commonly accepted. In the vicinity of several 

 of these domes are secondary centers of crystallization which have caused 

 minor domes. 



Harris believes, in the words of Washburne :^^ 



"That the amount of uplift of the strata is entirely inadequate to account 

 for the amount of space occupied by the salt plugs, some of which have been 

 penetrated by the drill nearly 3,000 feet without reaching bottom. He con- 

 cludes, therefore, that the salt cores are not laccolithic or pluglike intrusions 

 into the sediments, squeezed up from a great hypothetical salt bed in some 

 lower formation, but rather that they have grown by crystallization at the 

 places where they now occur and have not undergone much deformation. In 

 other words, they are great cylindrical concretions of salt 1,000 feet or more 

 across and over 3,000 feet high. He believes that the salt and the associated 

 hydrocarbons were gathered by meteoric waters which percolated through the 

 sedimentary strata and rose along the intersections of fissures, where the salt 

 was precipitated because of the decrease in temperature and pressure. The 

 decrease in pressure would cause but a negligible precipitation, practically 

 nothing. Temperature is somewhat more effective, but Lindgren^" says: 'As 

 the solubility of salt increases only slightly with increase of temperature (35.69 

 per cent at 10° C. ; 39.12 per cent at 100° ; 44.90 per cent at 180°), only the 

 increment could have been precipitated as the temperature of the ascending 

 current was lowered, and hence the quantity of primary salt required by this 

 hypothesis is incredibly large.' Let us assume that the solutions cooled as fast 

 as the normal underground head gradient, or about 21° C. in ascending 2,000 

 feet. This would precipitate about 2 per cent of the total salt in a solution 

 saturated at 100°. In other words, the salt cores would represent only about 

 2 per cent of the total amount of sodium chloride which had risen in the fis- 

 sures, the rest having been carried beyond the top of the cores and lost. The 

 salt cores in this country and Mexico number several hundred and their total 



5" C. W. Washburne : Trans. Am. Inst. Min. Engrs., vol. 48, 1914, p. 691. 

 et- Mineral resources of the United States (1913), p. 288. 



