CONTENTS. *i 



LETTER XXIV. 



Page 

 Inquiry respecting the Origin of Coal continued. ..Hypothesis proposed... Mosaic Account of the 



Deluge. ..Objections against. ..Universality of the Deluge. ..Changes thus effected... Vegetable 

 Matter disposed in Situations in which Coal now exists 246 



LETTER XXV. 



Inquiry whether the vegetable Matter was deposited at the Deluge under Circumstances favourable 

 to its Conversion into Coal. ..Bitumen alone not fitted for Fuel. ..Other Matters necessary to be 

 added. ..Peculiar Arrangement of the Particles 256. 



LETTER XXVI. 



Recapitulation. ..Apparent Agreement of the Hypothesis with the Economy of Nature 265 



LETTER XXVII. 



Mineral Charcoal. ..Opinions respecting. ..Subterranean Combustion of Pit-coal. ..Combustion of 

 Pyrites... Mineral Charcoal 272 



LETTER XXVIII. 



Pyritous Woods. ..Opinions of Dr. Hutton and Mr. Playfair... Igneous Origin. ..Aqueous Origin 

 examined 283 



LETTER XXIX. 



Petrifaction. ..Theories respecting. ..Substitution. ..Adopted by Walch, Kirwan, Daubenton, Four- 

 croy, &c... .Theory of Dr. Hutton and Mr. Playfair 295 



LETTER XXX. 



Theory of the Petrifaction of Wood proposed. ..Petrified Wood, silicious, calcareous, and alumi- 

 nous 300 



LETTER XXXI. 



Silicious Pebbles. ..Conjectures as to the Time and Mode of their Formation. ..Crystalline Fluid 

 of Reaumur. ..Agatine Nodules. ..Theory of their Formation. ..Opinions of Dr. Hutton and Mr. 

 Play fair... Aqueous Origin supported 311 



LETTER XXXII. 



Silicious Waters of Carlsbad. ..Of Iceland. ..Of Bath. ..Silicious Tufa of the Geyser.. .Vegetable 

 Calculi. ..Flint in the Epidermis of Plants 323 



LETTER XXXIII. 



Petrified Wood... Divided into silicious, calcareous, aluminous, &c. ...Silicious divided into sili- 

 cized Wood and silicized bituminous Wood. ..The latter, into calcedonic, agatine, jasperine, 

 and opaline 326 



LETTER XXXIV. 

 Calcedonic Wood... agatine... jasperized 331' 



LETTER XXXV. 

 Opaline Wood.. ,The Result of the Union of siHcious and soft bituminous Matter 337 



LETTER XXXVI. 



Evident Affinity between silicized, bituminous, or opaline Wood and Pitch-stone. ..Analysis of 

 opaline Wood... Of Pitch-stone.,, Similarity inferred , 34* 



