200 N. H. DARTON 



the first 50 feet — where the mean annual temperature is the under- 

 ground temperature — a depth of 4,300 feet would be required for 

 the spring water to become heated to 135° under ordinary condi- 

 tions. As the granite probably lies only 2,500 feet below the spring, 

 or 4,000 feet below the surface in the syncline south, this rate of 

 increase would indicate a source at least as low as the base of 

 the Deadwood formation. If the water is derived from the basal 

 sandstone of that formation, it passes underground in the outcrop 

 area in the high mountain slopes to the southeast, and becomes 

 heated in the bottom of the syncline a short distance south of Ther- 

 mopolis. In order to preserve this heat in its course to the outlets, 

 there must exist cavernous channels affording rapid flow, as heat 

 would be lost in slow percolation through the interstices of the rock. 

 It is possible also that the source of the water is much less deep, and 

 the heat may be due to deep-seated igneous rocks in this vicinity, 

 which have not yet cooled. As the nearest outcrops of igneous rocks 

 are in the Shoshone Mountains 40 miles west, it appears improbable 

 that there are any intrusions under the Thermopolis region. 



