Economic Geological Deposits. — Grosby. 261 
tricts; the great lenticular veins of pyrite along the Appalachian belt; 
the lenses of specular hematite on the James river in Virginia; and 
possibly some of the ureal lenses and sheets of magnetite belong here. 
(e) Neek i-tiiix. filling the interstices of agglomerates in volcanic pipes 
or necks. This is a recently, but not quite generally, recognized type. 
based upon the Rassick and Hull Domingo silver mines near Rosita, 
Colorado. These ore bodies or deposits are chimneys 20 to 100 and 4(1 
to 60 feet in horizontal section, in andesite and in Archean gneiss, re- 
spectively, filled chiefly with fragments of the wall-rock: and between. 
over, and replacing these fragments, the deposition of the ores has taken 
place. This mode of occurrence is. a priori, so probable that we may 
fairly suppose thai other and more important examples have been over- 
looked or misinterpreted. The resemblance of this type to the lenticu- 
lar crushed or shear veins is quite marked, and when we consider that, 
as in these cases, the necks of the craters and craterlets are naturally 
elliptical in outline and tend to occur in linear series along lines of fis- 
sure, the need of proceeding cautiously in distinguishing these two types 
is obvious. 
(f) Grenitic veins, filling the conduits of thermal springs. This type, 
which so far as 1 know, is proposed here for the first time, is suggested 
by the deposits of silver-lead ores of Red mountain, Ouray county, Colo- 
rado. The deposits traverse the mountain in an irregular way and are 
believed to mark the courses of old hot spring conduits. The wall-rock 
is silicified andesite, having been highly altered by the ancient thermal 
waters. This type differs from the true fissure veins much as a volcanic 
neck differs from a typical dike. /. e., the ascending ore solutions are 
more localized, and it is perhaps possible that at gfeater depths these 
conduit deposits pass into normal fissure veins. BelieYing as we do that 
mineral springs have played an important part in the formation of 
veins, the only occasion for surprise is that these creiiitic veins have not 
been more frequently identified and described: and it will be recalled 
that Sandberger finds here one of his chief arguments againsl the theory 
that veins are formed by ascending currents. It is probable that the 
conduits have, in most cases at least, been somewhat enlarged and 
modified in form by solution prior to filling. This assumption would 
be particularly safe when the wall-rock is limestone: but such modifi- 
cation must evidently be equally common with gash veins and other 
types having the normal sheet form. The cavities of mechanical and of 
chemical origin are thus combined, but gradation and transition forms 
are the rule everywhere in geology, and veins are no exception. 
(g) Cavern veins, filling cavities due to solution, including chambers. 
Hals, sheets, pitches, etc. This is the principal tV] I' veins filling 
spaces of dissolution. On a priori grounds i1 has strong claims to rec- 
ognition. Solution cavities or caverns are numerous, especially in 
limestones above the drainage level; and we often observe them in pro- 
cess of filling, especially with stalactite and stalagmite deposits. The 
chief difficulty is to find undoubted examples of ec »mic interest; 
nearly all the deposits formerh referred to this type having been proved 
