528 Capt. Newbold on a recent Fossil Freshwater Deposit in 



cesses by which fissures in rocks are lined and filled up with 

 minerals which we look in vain for in the enclosing walls; 

 geodes of calcedony and agate, with calc spar and crystals of 

 quartz and zeolite in the midst of calc spar. I have seen a 

 solitary and beautiful pyramidal hexagon of rock crystal, glit- 

 tering like a diamond in the whitest snow, in a mass of the 

 saccharine marble of Carrara. 



None of the shells have lost their carbonic acid, although 

 they have parted with most of their colour; and some are 

 quite empty as if imbedded but yesterday ; most have been 

 evidently entombed in a dead state. 



As no trap or other volcanic rock was at hand to account 

 for the silicification of this freshwater limestone, I proceeded 

 to examine the present deposit of the spring a few yards west 

 of the fossil bed. Its water I discovered to be slightly ther- 

 mal, having a temperature of 85° 3' Fahr., which is a few de- 

 grees above the mean temperature of the spot, isothermally 

 calculated; the height above the sea, as roughly approximated 

 by the boiling-point of water, is about 1250 feet, and the ave^ 

 rage temperature of the ordinary wells about 80° Fahr. 



The present deposit of these waters is a brownish-gray cal- 

 careous mud, about six inches thick, mingled with sand, im- 

 bedding similar freshwater shells and a minute specimen of 

 Paludina. Stems of grasses and leaves were also found in it ; 

 some of the latter apparently just decayed, while others are 

 blackened by carbonization; none were fossilized. 



Below the mud lay a deposit of nodular kunker, quite di- 

 stinct in character from that of the fossil bed, being white and 

 earthy, externally pulverulent or chalky, but internally com- 

 pact and hard. 1 did not observe any shells or plants in it. 

 The depth of this layer could not be ascertained for want of 

 leisure, and better instruments for digging under water than 

 a geological hammer. 



The water of the spring is tasteless, inodorous, and free 

 from gaseous bubbles ; and, instead of any free carbonic acid 

 gas, is slightly alkaline, turning reddened litmus paper into a 

 faint greenish-blue; oxalate of ammonia and muriate of ba- 

 ryta produced a considerable white precipitate. That from 

 the muriate of baryta effervesced with dilute nitric acid, show- 

 ing the precipitate to be carbonate of lime. A thin slice of 

 gall-nut, suspended in the water, detected a trace of iron. A 

 minute portion of silica remained after evaporation. 



The present layer of mud then, as we have just seen, is 

 more of a mechanical deposit than the subjacent white kun- 

 ker, which is evidently a chemical precipitate, and concre- 

 tionary in character, while that which has fossilized the shells 



