364 A. Vaughan Jennings — Geology of Bad Nauheim. 



Ibeyond the reach of the borings, where a high temperature prevails 

 and intense chemical reaction is still in progress. 



The soluble saline constituents again oifer us much the same 

 problem. The physiographic changes of Tertiary times must have 

 been slow, gradual, and constantly varying, and the alternations of 

 marine, brackish, and fresh-water conditions probably resulted in 

 •a large residue of salt in the strata then being deposited. The 

 minerals present in such formations as the Blattersandstein certainly 

 suggest this ; but whether or not such a residue is still present, a vast 

 amount has probably been washed down into the subjacent rocks. 



If this be considered insufficient as a source of the present regular 

 supply, one is obliged to fall back again on the supposition that 

 there exist below the deepest borings either great salt beds or 

 a stratum of immense chemical activity. 



Turning from the question of chemical contents to that of physical 

 conditions, one meets again with questions more easily asked than 

 answered. The most obvious are the cause of the high temperature 

 of the waters, and the reason for their expulsion. As to the former 

 one might suggest a ver^'^ great depth of origin, but we have no 

 evidence that these springs rise from a stratum deep-seated enough 

 to account alone for their heat. Mechanical pressure, added to such 

 strong chemical activity as may exist at a relatively high level in 

 the earth's crust, even locally, might also account for it in part. 

 In addition to such conditions, however, we have to remember the 

 proximity of great basaltic masses which were themselves in a state 

 of fusion at a period not, geologically speaking, so very remote. 

 As the presence of thermal springs is so commonly correlated all 

 the world over with the existence of comparatively recent volcanic 

 action, it is probable that we are safe in here regarding the present 

 temperature of the brine-springs as residual from volcanic times. 



With respect to the cause of the expulsive force with which the 

 waters of the two great 'sprudels' are thrown up, it seems that 

 the German geologists, as a rule, are inclined to follow their great 

 leader Bunsen, who studied the district early in the century and 

 ascribed the effects entirely to the freeing of the carbon dioxide. 



That the sudden liberation of gas, with the diminishing pressure 

 as the water rises in the tube, must have a very great effect in 

 producing the violence of the final escape, cannot be denied ; but this 

 does not necessarily justify a neglect of hydraulic pressure altogether. 

 The great drainage area of the Taunus, its large rainfall, and the 

 fissures and cavities it contains all seem to suggest the probability 

 of a very efficient hydraulic system at work ; and the history of 

 the Great Sprudel, where the first outburst only occurred some years 

 after the construction of the boring, as a consequence of heavy rains, 

 would appear to support this view. 



Also, as Professor Cole has pointed out to me, why neglect 



* residual pressure ' ? Just as we may attribute high temperature 

 to the survival, ia extremis, of volcanic conditions, so the tectonic 



* Alpine ' movements are not to be regarded as entirely dead, but the 

 remaining strains and pressures in the earth's crust may still be 

 factors in the ejection of these heatedvvaters. 



