So The Naturalist in Nicaragua 
fissures of rocks already greatly heated, and the cooling of 
these rocks has been prolonged over thousands of years, 
during which the lodes have been exposed to every degree of 
heat, from that of fusion to their present normal temperature. 
During the slow upheaval and denudation of the lodes, they 
have been subjected to various chemical, hydro-thermal, and 
aqueous agencies, by which many of their contents have been 
re-arranged and re-formed, new minerals have been brought 
in by percolation of water from the surrounding rocks, and 
possibly some of the original contents have been carried out 
by mineral springs rising through the lines of fissures which 
are not completely sealed by the igneous injection, as the 
contraction of the molten matter in cooling has left cracks 
and crevices through which water readily passes. 
5. Some of the fissures may have been re-opened since 
they were raised beyond the reach of molten matter, and the 
new rent may have been filled by hydro-thermal or aqueous 
agencies, and may contain, along with veinstones of calcite 
derived from neighbouring beds of limestone, some minerals 
due to a previous igneous injection. Crevices and cavities, 
called vughs by the miners, have been filled more or less com- 
pletely with crystals of fluor spar, quartz, and various ores 
of metals from true aqueous solutions, or by the action of 
super-heated steam. 
6. By these means the signs of the original filling of many 
mineral lodes, especially those of the baser metals, have been 
obscured or obliterated; but in auriferous quartz lodes both 
the metal and the veinstone have generally resisted all these 
secondary agencies, and are presented to us much the same 
as they were first deposited, excepting that the associated 
minerals have been altered, and in some cases new ones 
introduced, by the passage of hot springs from below or 
percolation of water from the surface. 
