554 



SCIENCE 



[N. S. Vol. XL VI. No. 1197 



conspicuous hill, some througii recent solu- 

 tion of the salt making- a depression, and some 

 having little or no effect on the surface, the 

 salt core of some lying at slight depth and of 

 others at great depth — it would appear that if 

 they are due to intrusion, the igneous rock 

 should have been found in some of them. 



Furthermore, in areas of igneous activity in- 

 trusions have various forms, dikes being com- 

 mon, but salt domes are sharply localized, 

 more or less equi-dimensional laterally, in 

 length and breadth rarely measuring over tvro 

 or three miles or less than one half mile. Al- 

 though in an area underlain by a great thick- 

 ness of unconsolidated strata intrusions may 

 differ somewhat from those of other areas, still, 

 since the country rock, being unconsolidated, is 

 more likely a body of water than if it had been 

 cemented into stone, it seems quite unreason- 

 able to assume that either intrusions or see- 

 ondai-y deposits made by circulating waters or 

 gases emanating from them would be similar 

 in form and size and short in lateral dimen- 

 sions. The fact that salt domes are found on 

 the northwest and southwest coasts of the Gulf 

 of Mexico, and that, due perchance to more 

 consolidated rock, igneous intrusions are com- 

 mon in territory between, invites investigation 

 to determine whether or not gradation phases 

 may be found between salt domes and intru- 

 sions. Such phases, however, seem to be poorly 

 developed and the series, if there is one, in- 

 complete. Also, although intrusions have 

 made dome structures and hills on the sur- 

 face in many parts of the world, no evidence 

 of an overlying salt core seems to have been 

 found. 



Are the domes due, as has also been sug- 

 gested, to forces of crystallization acting in 

 some such way as they do in the growth of 

 concretions, the salt being taken from satu- 

 rated solutions and collected around some nu- 

 cleus by molecular attraction? Ordinarily 

 salt does not seem to behave in this way and 

 the associated great deposits of dolomite, gyp- 

 sum and other secondary substances would 

 seem too much to ascribe to a kind of mass ac- 

 tion not controlled by some other set of forces 

 operating at or underneath the locus of salt- 



dome growth. The apparent lack of concentric 

 structure and of small salt concretions, and 

 the presence of certain minerals, such as 

 sulphur and copper ores, seems to point to a 

 deep-seated cause for the domes. 



May the salt domes be due to a buckling and 

 flowage of one or more beds of rock salt lying 

 at great depth, as has been suspected concern- 

 ing European salt domes or more indirectly 

 to some process of isotatic adjustment? If 

 so some of the salt cores should be connected 

 below with the parent stratum or strata of 

 rock salt, and the average mass of salt per- 

 haps much greater than if it had developed in 

 some other way. However, since the country 

 rock is largely unconsolidated, and, on the 

 whole, homogeneous, and the surface is smooth 

 and horizontal, it would seem rather improb- 

 able that the bodies of salt could have been pro- 

 duced through differential pressure, though it 

 must be admitted that a small stress difference 

 operating for a very long time may accomplish 

 a great deal, and once started the process might 

 be somewhat self-accelerating. Also the asso- 

 ciation of salt, dolomite, gypsum, sulphur, 

 copper, etc., suggests a Permian source. As a 

 matter of fact, however, the few determina- 

 tions of specific gravity of the country rock 

 that have been made indicate that it weighs in 

 its natural wet state no more than salt, if in- 

 deed as much, and it seems very improbable 

 that there has been any considerable horizon- 

 tal thrust pressure. 



In any case from what has been learned by 

 deep boring and from the various conceivable 

 possibilities as to salt dome origin, it seems 

 probable that the known upper portions of 

 salt cores are underlain with (a) more salt, 

 (6) clay and sand, or (c) igneous rock. Al- 

 though it is possible that the clay and sand 

 strata through which the salt rises, differ more 

 or less markedly from it in specific gravity, 

 the surprisingly little information available on 

 the subject indicates that in their natural state 

 the salt is appreciably the heavier. The writer 

 has tested seven samples of common sandy clay 

 and clayey sand from the Gulf Coast region, 

 and the results indicate that although the spe- 

 cific gravity varies considerably, it is not far 



