CONTENTS. 
XI 
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 to 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 
272 
Pyrites... Mineral Charcoal, . . 
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 Aluminous, 306 
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. 
Playfair.. .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 Silicized 
Wood and Silicized Bituminous Wood. ..The latter, into Calcedonic, Agatine, Jasperine, and 
Opaline, . . . . . . . . . . . . . _ _ _ _ _ _ _ 323 
Calcedonic Wood . . . Agatine . . . J asperized. 
LETTER XXXIV. 
331 
LETTER XXXV. 
Opaline Wood.. .The Result of the Union of Silicious 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, .. .. .. .. .. .. 345 
