446 KIRK BRYAN 



The emergence of the water through the Polk Creek and 

 Missouri Mountain shales into the Hot Springs sandstone requires 

 no special hypothesis, if the thrust fault with associated jointing 

 and fissuring which seem to be indicated by field relations is 

 granted. 



By reference to the table of temperatures, page 430, it will be seen 

 that Big Chalybeate Spring has a temperature above normal. In 

 chemical composition the water is of the calcium carbonate type, 

 and of about the same mineralization as the Hot Springs waters, 

 differing mainly in having less silica, (see table, page 427.) It has a 

 strong and according to local observers a steady flow, which 

 measured by H. D. MitchelP was found to be 186 gallons per 

 minute. This spring lies 5I miles northeast of Hot Springs on the 

 northwestern flank of the mountain which is the extension of the 

 North Mountain syncline. The spring apparently arises from 

 the Polk Creek shale in the flat valley of a tributary of the West 

 Branch of Gulpha Creek. If the water is derived from rainfall on 

 the Bigfork, which saturating the chert arises through a fracture 

 in the shale, it is difiicult to account for the abnormal temperature, 

 18° above the mean annual air temperature, unless an uncooled 

 igneous plug is postulated for this spring also. The contact of 

 the shale with the chert on the west is less than one-tenth of a mile 

 away and less than 20 feet above the spring. The difference in 

 head seems insufficient to force the water to travel to depths and 

 return. On the other hand, southwest of the spring three-fourths of 

 a mile is another anticlinal area of Bigfork chert which has its 

 contact with the shale at elevations between 620 and 820 feet. 

 It might be postulated that water from this area would flow north- 

 west under the syncline and emerge at Big Chalybeate Spring. 

 This hypothesis has the advantage that a depth of 1,000 to 2,000 

 feet would be attained, and this depth would doubtless be sufficient 

 to account for the temperature of the water. The spring, however, 

 has an elevation between 620 and 640 feet. For this postulate, 

 also, there appears to be a lack of hydraulic head. The origin of 

 Big Chalybeate Spring is then as much an unsolved problem as the 

 origin of the Hot Springs, but because the waters are both thermal 



' J. C. Branner, op. cit., p. 29. 



