a Theory of the 'Earth. 149 



This refearch is particularly interefting when thefe excava- 

 tions are extended below the level of the fea.- 



6. Marl-pits ; their external appearances ; whether they 

 contain fhells, and of what kinds; extent of their beds, and 

 their thicknefs ; analyfis of them, at lead with acetous acid ; 

 the ufes for which the marl is employed. 



7. Clay-pits; quarries of lime-ftone, gypfum ; mines of 

 coal, &cc. 



8. Whether the plains exhibit at their furface, or contain 

 in the interior parts of the earth, veftiges of marine bodies, 

 petrified wood, bones or other fubftances foreign to the foil 

 and to the country. 



o. Internal temperature of the earth, afcertained either by 

 direct experiments, or by obferving that of the deeper! wells 

 or cellars ; or by that of fprings, which do not freeze in win- 

 ter, and which remaining cool in fummer, feem to come 

 from the greateft depths. 



10. Whether any fa£t can be obferved which might oblige 

 us to have recourfe to the hypothefis of a central fire. 



11. Bafons furrounded by hills or mountains; whether 

 they feem to have been formerly filled with water ; whether 

 the water appears to have been freih or fait ; whether any 

 thing indicates the period of its retreat, and if there are any 

 traces of the apertures through which it efcaped. 



CHAP. VIII. 



Observations to be made on Rolled Pebbles. 



1. The nature- and fize of thofe found in any particular 

 dillrict. 



2. To examine, above all, whether there is any kind 

 which may be confidered as peculiar to that diftri&, or which 

 may be proper to charafterife it ; or even, whether the ab- 

 fence of any kind or clafs might not be fuflkient to form 

 that chara&er. 



Lj V Wlit- 



