CONTENTS. 

 LETTER XXXVII. 



Pasr 



-"Semi-opal.. .Experiment on Semi-opal of Telkebanya... Opal. ..Its wonderful Properties. ..Analysis 

 by Klap roth.. .Horn-stone and Flint ^53 



LETTER XXXVIII. 



Calcareous Wood. ..Lime frequently held in solution, in 'Water... Various Forms of Deposition... 

 C&nfetto di 'Tivoli... Quarries of tufaceous Stone. ..Stalactitic Caverns 360 



LETTER XXXIX. 



Calcareous fossil Wood. ..Where found. ..Calcareous Wood of Oxfordshire. ..Dorsetshire. ..Somer- 

 setshire. ..Analysis. ..Mixture of Spar and Bitumen... Calcareous Wood of New South- Wales... 

 Aluminous Wood, &c. 37+ 



LETTER XL. 



Metallic Fossil Wood. ..Bog Iron Ore. ..Fossil Wood, impregnated with Iron. ..With Copper, &c. 384- 



LETTER XLI. 



Variety of petrified Woods. ..Proceeding from original natural Difference*.. From the Labours of 



Man 391 



LETTER XLII. 



Varieties of petrfiied Woods continued. ..Those proceeding from the Operationsof Insects. ..Starry 

 Stone of Chemnitz... Teredo navalis 4/1! 



LETTER XLIII. 



Secondary vegetable Fossils. ..In Schisti...In Sand-stone. ..In calcareous Strata. ..In argillaceous 



Nodules 4u 



LETTER XLIV. 



Great Difficulty of ascertaining even the Genera of the Plants which are thus preserved... Dorsi- 

 ferous Plants and Cacti most common > ., 4j@ 



LETTER XLV. 



Fossil Stems of Plants. ..Of the Reed, &c....Fo$jz7mn incognitum , 424, 



LETTER XLVI. 



Remarks on Leaves contained in Nodules. ..Impressions of the same Side of the Leaf on each No- 

 dule. ..Accounted for by Jussieu, Schultz, &c... .Explanation proposed 431 



LETTER XLVII. 



'Fossil Flovrers... Their Existence doubtful. ..Fossil Seeds and Seed-vessels. ..Fossil Fruits. ..Fossil 

 Loaves 433 



LETTER XL VIII. 

 Conclusion ...., 45^ 



