228 Prof. B. Silliman junior on the 



the slime which the rains have washed down into the circular hollows 

 adjacent these improvements, thus rendering them water-tight. 



The stage-road to Nashville runs over this upper plateau all the 

 way from Salt River, with unimportant exceptions in the valleys of 

 small rivers, until it reaches the valley of the Cumberland River, 

 over 150 miles. Within some ten or fifteen miles of Nashville the 

 traveller again descends by a similar gradual declivity full 350 feet 

 to the banks of the Cumberland River, on which the city of Nashville 

 stands. Here again the observer detects the oldest silurian rocks, 

 compact blue crystalline limestones, in which the fossils are almost 

 obsolete, and unfossiliferous sandstones, alternating with them. 



The Mammoth Cave of Kentucky is situated in the midst of the 

 great upper plain, and descends from the uppermost rocks to those 

 which are nearly or quite on a level with the Ohio. Dr John Locke 

 of Cincinnati has measured the depth of the cave barometrically, and 

 is said to have determined it to be 325 feet below the hotel, and the 

 level of the rivers of the cave to be the same as that of Green River, 

 with which the subterranean rivers or lakes rise and fall in times 

 of flood ; so that there is no doubt of a connection existing be- 

 tween Green River and these curious subterranean pools over which 

 the explorer passes in mysterious silence many miles from the en- 

 trance, and at least 300 feet from the upper surface. Unfortunately 

 our party was unprovided with barometers, and the measurement of 

 Dr Locke remains therefore as the only evidence we have of the rela- 

 tive levels of these subterranean waters and Green River. 



One atmospheric phenomenon attracted our attention and tasked 

 our ingenuity for a satisfactory explanation. I will mention the 

 fact and our solution of it. If the external air has a temperature 

 above 60° Fahr., the observer, on approaching the mouth of the 

 cave, is met by a blast of cool air blowing outward from the mouth ; 

 and if the external temperature is high, say 90° Fahr., the blast 

 amounts to a gale. It is almost impossible, indeed, where the dif- 

 ference of temperature, either way, is considerable, to make the 

 entrance of the cave with a lighted lamp ; unless carried with an 

 experienced hand it is surely extinguished. In hot weather, this 

 contrast of temperature and its accompanying blast of air, are at first 

 quite overpowering, and you feel as if immersed in a cold bath. 



If the air without has a temperature of 59°-60° no current is 

 observed, and the flame of a lamp held in a favourable position in- 

 dicates none. It immediately occurred to me, that there must be 

 two currents — one above of warmer air, passing inward, and one be- 

 low of colder air, passing outward, and the reverse, but experiment 

 soon satisfied me that this was not the case. Only one current 

 could be discovered, and on inquiry of the very intelligent guide, 

 " Stephen," who is well known for his remarkable powers to all who 

 have been at the cave, I found that this phenomenon had attracted 

 his attention, and that he was satisfied from many observations, that 



