SILICIFIED FOSSILS OF THE CHALK. 181 



intermittent boiling fountains, called the Geysers, 

 in Iceland, are well known ;* and in New Zealand 

 this phenomenon is exhibited on a still grander 

 scale. From the crater of the volcanic mountain 

 of Tongariro, which has an elevation of 6,200 

 feet above the level of the sea, jets of vapour and 

 streams of boiling water highly charged with silex 

 are continually issuing forth, and dashing down 

 the flanks of the volcano in cascades and torrents, 

 empty themselves into the lakes at its base. As 

 the temperature of the water diminishes, siliceous 

 sinter is deposited in vast sheets, and incrustations 

 of flint are thrown down on the extraneous sub- 

 stances lying in the course of these thermal 

 streams. Silex is also precipitated by the boiling 

 waters, in the state of stalagmitic concretions, and 

 in nodules resembling in colour and solidity the flints 

 of the English chalk.f The complete impregna- 

 tion, and silicification (i. e. transmutation into flint) 

 of organic bodies is referable to an agency of this 

 kind, as the eminent American mineralogist, Mr. 

 Dana, has clearly shown.;}; 



SiLICIFIED FOSSILS OF THE CHALK. 111 many 



instances the organic remains in chalk flints, are 



* " Wonders of Geology," vol. i. p. 84. 



t Dr. Dieffenbuch's " New Zealand." 



t " American Journal of Science," for January 1845. See also, " Notes 

 of a Microscopical Examination of Chalk and Flint," by the Author. Annuls 

 Nat. Hi.it. 1845. 



