S32 Hints for the Formation of 
29. The origin of volcanic ashes, pozzolanas, tarras, 
tufas, &c. 
30. The origin of pumice-stones : whether they are 
granites or feld- spars, asbestos, phrenites, deodalites, or 
potter’s clay more or less ferruginous, or, lastly, the re- 
mains of the decomposition of pit-coal. 
30. A. Whether, as M. de Fiehtel believes, the ac- 
tion of the fire of volcanoes may augment the fusibility 
of feld- spar, and change it, in the same manner as quartz, 
into real zeolite. 
31. Nature of the obsidians or volcanic glass : whe- 
ther they be really glass, and the results of a complete 
fusion ; or whether they are not rather stones of a vitre- 
ous appearance, and which have not been exposed to the 
action of a heat sufficiently strong to fuse them. 
32. Whether there exist ancient kinds of lava, which, 
as related of those of Ischia, are susceptible of being 
heated by the moisture of rain and fogs, which would 
support the conjecture of No. 14. 
0. Observations to be made on Hills and Mountains 
which are doubted to have been really Volcanoes. 
33. The form, elevation, and other dimensions of the 
hill or mountain, the volcanic origin of which may ap- 
pear doubtful. 
34. Situation of its strata. To ascend to the summit 
of those which are inclined ; to examine whether they 
have not a crater, or vestiges of one. 
3 5. To observe, above all, whether, in departing from 
the most elevated point, there are found strata diverging 
in all parts, and proceeding from that point as a centre. 
36. To study the characters of the stones which have 
been exposed to the action of the fire, in order to distin- 
guish them from other porous stones, such as glandulous 
stones or amygdaloides* 
