68 NEW YORIv STATE MUSEUM 



Up within cavities of originally small compass, such as joints and 

 the openings along bedding planes. 



An example of the accumulation of salt and gypsum by the work 

 of ground waters is found according to G. D. Harris^ in the so 

 called " Five Islands " or " Salt Islands " of Louisiana which rise 

 as dome-shaped hills above the low coastal plain of the gulf. The 

 domes are not due apparently to differential erosion but have been 

 actually uplifted en masse, so that the strata dip away froim their 

 centers on all sides only to become horizontal as the plain level is 

 reached. Their uplift has been ascribed previously to different 

 agencies, including gas pressure, water under a great head, and to 

 deep seated igneous masses which are working toward the surface. 

 Harris finds that the domes occur at the intersections of master 

 faults and thinks the faults have served as channels for the ascen- 

 sion of saline waters from great depths. With temperatures cor- 

 responding to their source in the interior at the start the waters 

 would rise throughout the faulted strata and be compelled to pre- 

 cipitate their salts as they become cooler on their way. The solvent 

 power of water for sodium chlorid decreases most rapidly between 

 the temperature of i8o° and that of 120° C. so that the precipita- 

 tion of this salt would take place in greatest amount at considerable 

 depths. The tendency therefore is to form a cone which, slender 

 at first and pressing against the surrounding strata, would grow 

 broader and longer by deposition at the base. The force of crystal- 

 lization, it is thought, might move the mass upward spreading out 

 the strata 011 all sides. With the deposition of salt tlie power of 

 holding calcium sulfate in solution increases until the salinity is 

 reduced to about 14 per cent, after which it rapidily decreases. Cool- 

 ing of the solution down to about 40° C. also increases the solu- 

 bility. The formation of gypsum would take place accordingly near 

 the surface, and it is noted that the gypsum of Louisiana and Texas 

 usually oiccurs above the salt. 



This hypothesis involves a striking, if not a novel, application of 

 the force of crystalliization to the origin of such deposits. It seems, 

 however, to meet the peculiar conditions that surround the occur- 

 rence of salt and gypsum in the gulf region (as well as in a few 

 localities elsewhere) conditions which are difficultly explainable by 

 the more common method of deposition from sea water. While 

 there is, thus, much in its favor from a geologic standpoint, there is 

 also need of more knowledge of the physical principle oin which 

 its validity ultimately depends, 



-^ Econ. Geol. 1909. 4:12. 



