510 OBSERVATIONS ON THE FORMATION^ 
tals of the various lead-spars ; but most frequently, and 
abundantly, with crystals of the carbonate. Indeed, speci- 
mens sometimes occur with nearly all the different spars, 
distinctly marked in close approximation. I have in my 
own possession a specimen 24 inches square, and 2 inches 
deep, which has in perfection every variety found here^ 
except the sulphate of lead. 
IV. The various lead- spars occur most abundantly, 
and in greatest perfection, in those very situations where 
we would naturally expect to find them, on the supposition 
of their being formed from the decomposition of the galena ; 
namely, in those veins which are open, and full of druses, 
and especially where the galena lies in detached masses, sur- 
rounded either entirely, or in part, by such vein-stuff as is 
readily permeable to water and air^ agents, of course, 
quite essential to the production of the change in question. 
V. In situations where it has been necessary to cut out 
solid rock in the vicinity of galena, the walls of a cavity so 
formed have been found, in a few years after, to be stud- 
ded over in various places with minute crystals of carbon- 
ate of lead : also, the solid stones, which are sometimes left 
impacted on various parts, on being brought to day, some 
years after, are found to be covered with the same incrus- 
tations, conveyed there, no doubt, by the agency of water. 
To the same point, I may mention, that the vein-stuff, 
which it is necessary to bring to bank, and which, when 
brought up, has no appearance of being any thing but 
stones and rubbish, is, after being exposed for a few years, 
found to be covered with an incrustation of minute crystals 
of carbonate and phosphate of lead, ^vhich had not been 
observed when first brought to bank, owing to the very 
comminuted state in which it then existed. 
