78 ° The Naturalist in Nicaragua 
peated re- opening and filling of the same fissure; but never, 
in quartz veins, a regular filling up from the sides towards 
the centre, as in veins produced by deposits from springs. 
Quartz veins extend sometimes for miles, and it is necessary 
to suppose on the hydro-thermal theory that the fissures 
remained open sufficiently long for the gradual deposition of 
the veinstones, without the soft and shattered rocks at their 
sides falling in, nor yet fragments from above; although — 
there are many lodes, fully twenty feet in width, filled entirely 
with quartz and mineral ores, without any included fragments 
of fallen rocks, and nowhere showing any trace of regular 
deposition on the sides. The gold also found in auriferous 
lodes is never pure, but forms various alloys of gold, silver, 
copper, lead, iron, and bismuth; and no way is known of 
producing these alloy Ss except by fusion. 
It is true that mineral veins contain many minerals that 
could not exist together undecomposed with even a moderate 
degree of heat; but it is only here contended that the original 
filling of the lodes was an igneous injection, not that the 
present arrangement and composition of all the minerals is 
due to the same action. Since the lodes were first filled they 
have been subjected to every variety of hydro-thermal and 
aqueous influence; for the cooling of the heated rocks must ~ 
have been a slow process, and undoubtedly the veins have 
often been the channels both for the passage of hot water and 
steam from the interior, and of cold water charged with 
carbonic acid and carbonate of lime from the surface, and 
many changes must have taken place. Auriferous quartz 
veins have resisted these influences better than others, 
because neither the veinstone nor the metal is easily altered, 
and such veins therefore form better guides for the study 
of the origin of mineral lodes than fissures filled with calc 
spar and ores of the baser metals, all readily dissolved and 
re-formed by hydro-thermal agencies. Our mineralogical 
museums are filled with beautiful specimens of crystals of 
quartz, fluor spar, and various ores deposited one on the 
other; and the student who confines his attention to these is 
naturally led to believe that he sees before him the process 
by which mineral veins have been filled. But the miner, 
working far underground, knows that such crystals are only 
found in cavities and fissures, and that the normal arrange- 
