CONTENTS. 
Xll 
LETTER XXXVII. Page 
Semi-opal...Experiment on Semi-opal of Telkebanya...Opal...Its wonderful Properties...Analysis by 
• Klaproth... Horn Stone and Flint, .. .. ■■ •• •• •• 
LETTER XXXVIII. 
Calcareous Wood.. .Lime frequently held in Solution, in Water... Various Forms of Deposition... 
Con/e^o di TwZi... Quarries of Tufaceous Stone. ..Stalactitic Caverns, 3^° 
LETTER XXXIX. 
Calcareous Fossil Wood.. .Where found... Calcareous Wood of Oxfordshire, Dorsetshire, Somerset- 
shire...Analysis...Mixture of Spar and Bitumen...Calcareous Wood of New South Wales... 
.... 374 
Aluminous Wood, &c. 
LETTER XL. 
Metallic Fossil Wood...Bog Iron Ore...rossil Wood, impregnated with Iron...With Copper, &c. 384 
LETTER XLL 
Variety of Petrified Woods... Proceeding from Original Natural Difference... From the Labours 
of Man, 
LETTER XLII. 
Varieties of Petrified Woods, continued... Those proceeding from the Operations of Insects... Starry 
Stone of Chemnitz... Teredo Marina, 
LETTER XLIII. 
Secondary Vegetable Fossils...In Schisti...In Sand Stone...In Calcareous Strata...In Argillaceous 
Nodules, 
LETTER XLIV, 
Great Difficulty of ascertaining even the Genera of the Plants which are thus preserved... Dorsiferous 
. .... 416 
Plants and Cacti most common, 
LETTER XLV. 
Fossil Stems of Plants... Of the Reed, iic...Fossilium Incognitum, 424 
LETTER XLVI. 
Remarks on Leaves contained in Nodules...Impressions of the same Side of the Leaf on each 
Nodule... Accounted for by Jussieu, Schultz, &c...Explanation proposed, 
LETTER XLVII. 
Fossil Flowers... Their Existence doubtful.. .Fossil Seeds and Seed Vessels... Fossil Fruits, • . • • 436 
LETTER XLVIII. 
455 
Conclusion, 
