512 OBSERVATIONS ON THE FOllMATION 
water, or of air, or both ; the oxide of lead will combine with 
the sulphuric acid, to form sulphate of lead ; carbonic acid 
(which is constantly present in water) may combine with 
another portion of oxide of lead, and will form the carbon- 
ate ; this last will be pure, dark, of various shades of black, 
according to the quantity of decomposed galena present. 
If oxides of iron or copper be present, they v/ili communi- 
cate to the crystals as they form, various shades, according 
to their quantity. What are called the new minerals, 
namely, combinations of carbonate and sulphate of lead, in 
various proportions, can be easily conceived to occur from 
being simultaneously formed, and combining as compound 
crystals. As to the phosphates, I presume the acid must 
be derived from the surface ; and what countenances this 
opinion is, that the best pliosphates have been found either 
not far from the surface, or in veins which seem to have a 
pretty free communication with the surface, and to have a 
plentiful supply of water. I am aware that there are con- 
siderable quantities of apparently solid galena, which, on 
being broken, are found to contain masses of lead-spars, 
of various sizes and forms, interspersed through them, 
and which would seem as if necessarily formed contem- 
poraneously with the consolidation of the galena. This 
seemed to me, for some time, to have been in reality the 
case ; and, therefore, that an exception must be made to the 
general truth of the theory I am endeavouring to advocate. 
But now^ I conceive that this difficulty may be easily sur- 
mounted, by supposing the galena to have been consoli- 
dated, with a number of these small cavities in it. Galena 
in higher situations, being transformed in the manner which 
I have supposed, the water which permeates the strata may 
take up the carbonate of lead, and, in its course through 
the neighbouring parts of the same, or other strata, may 
deposit it, in a crystalline form, in situations, where, by rest 
