BLACK ROOK MOUNTAINS. 793 



superposition of rhyolite over basalt, is clue to a general faulting and frac- 

 turing of the beds at the time of, or since the outflow of the basalts, and that 

 in the one instance observed, where rhyolite seemed actually to overlie 

 basalt, the latter might have poured out between previously formed rhyolite 

 beds, as undoubtedly occurs at times in sedimentary rocks. 



It is interesting to observe in this connection that the abundant hot 

 springs which occur in these hills are almost invariably found on the edge 

 of the desert along the western foot of the mountains. Of these, the group 

 of Warm Springs, locally known as the Double Hot Springs, about 6 miles 

 from the southern point, consists of two adjoining pools about 15 or 20 feet 

 in diameter, and apparently about as many deep, with other smaller pools 

 in the vicinity, from all of which there is only a small overflow of water. 

 The water has no perceptible taste, but it has a very high temperature, that 

 of the larger pools at the surface being- 165° to 168° F., while one of the 

 smaller pools gave a temperature of 171° (78° C). Probably below the 

 surface the temperature is very near the boiling-point. 



This region possesses a weird interest, not only from its peculiarly 

 desolate physical character, being, with the exception of the. hot eprings 

 above mentioned, entirely without water, and utterly devoid of vegetation, 

 not even supporting a growth of the almost ubiquitous sage-brush (Arte- 

 misia tridentata), but also from the large development of the more unusual 

 accompaniments of volcanic rocks : concretions and geodes of chalcedony 

 and agate are present in great quantity and of most varied forms and colors, 

 while the occurrence of a persistent bed of decomposed basalt, perfectly 

 honey-combed with amygdaloidal cavities filled with green earth, the 

 fissures of the more compact portions being covered frequently with a thin 

 dendritic coating of oxide of iron, has been the cause of leading hundreds 

 of ignorant but enthusiastic miners into the belief in the presence of valuable 

 argentiferous minerals. At the time of our visit in 1867, this belief, fostered 

 by fabulous reports, spread abroad in part by ignorant, in part doubtless 

 by designing, persons during a series of years previous, the fear of hostile 

 Indians and the inaccessibility of the region rendering trustworthy accounts 

 difficult to obtain, had culminated in the establishment of a mining-town, 

 called Hardin City, just north of the limits of the map, near some springs 



