GEOLOGY OF THE PROVINCES OF BARAHONA AND AZUA. 193 



Springs of pure water appear to be exceptional in this region. Most of 

 those near Lake Enriquillo are sufficiently free from sulphur to be potable. 

 Curiously enough there is a fresh-water spring less than 20 meters from 

 the strong salt spring on the beach a short distance south of Barahona. 

 Springs of pure water are reported near Rio del Medio on the trail between 

 Constanza and Tubano, at Las Yayas, and there are doubtless others. 



The springs near Lake Enriquillo are unusual in several respects and 

 deserve study. They border the basin on both the north and south sides, 

 emerging from the lower edge of the bench of coral limestone which 

 fringes the mountains. This uniformity in position is probably not due to 

 any fundamental structure and has no bearing on the origin of the springs. 

 The springs have their origin in the older limestone that forms the moun- 

 tains against which the coral limestone has been deposited. The coral 

 rock is of recent age and is very porous. It probably merely conceals the 

 source of the water, which doubtless flows out of the ro^k on which it lies, 

 percolates through it, and emerges at its outer edge. The springs probably 

 antedate the coral rock and may be of diverse origin, an assumption borne 

 out by the marked differences in the sulphur content of springs that are 

 close together. Hydrogen sulphide is so generally a constituent of the 

 spring waters in this region that its presence in most of the springs does 

 not imply a common origin. Although the coral limestone conceals the 

 source of these springs, many of them are probably tubular solution cavities 

 in the limestone of the mountain, like those at El Puerto near Las Matas 

 and at other places in the region where the source of the water is more 

 evident. A number of these springs are large, forming brooks of con- 

 siderable size. The sulphide in the water does not appear to be harmful 

 in irrigation, for which the water is extensively used. 



Chemical Composition of Spring Water. 



Mineral analysis and classification of spring water (dug ditch) from Campo el Puerto, 



Province of Azua. 



[Samples collected by C. P. Ross, May 18, 1919; analyzed by Addie T. Geiger.] 



Constituents. Parts per million. 



Silica (SiO) 2 18 



Iron (Fe) .29 



Calcium (Ca) 92 



Magnesium (Mg) 18 



Sodium and potassium (Na K) (calculated) 12 



Carbonate radicle (C0 3 ) .0 



Bicarbonate radicle (HC0 3 ) 282 



Sulphate radicle (S0 4 ) 27 



Chloride radicle (CI) 49 



Nitrate radicle (NO a ) Tr. 



Organic matter 5 . 2 



Total dissolved solids (180° C.) 350 



