234 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
remarkable that the shell is still made up of the aragonite of which it was 
originally composed. In the open cut near the Smuggler shaft the writer 
found a similar fossil gasteropod in hard ore. Both this fossil and the 
inclosing rock are largely made up of zine and lead sulphides and carbon- 
ates, the composition of the fossil being exactly like that of the rock. The 
base of the fossil was embedded in hard ore like itself, but around the 
remainder the ore had been softened and so had fallen away, leaving 
the fossil projecting from the walls of the cut. 
On the other hand, some of the metallic sulphides were deposited in 
preexisting cavities. In some of the ores on West Aspen Mountain, which 
occur along north-south faults, the galena is found as a crust which covers 
the fragments of a breccia, and often the mineralization has been so partial 
that irregular cavities are still left unfilled, the walls of which are coated 
with galena. In solid dolomite and limestone the sulphide often occurs in 
small nonpersistent seams, which have the aspect of veins, and the 
occurrence of argentiferous tetrahedrite or tennantite along crevices in 
heavy spar, forming the so-called crisscross spar, has already been 
described. 
CAUSE OF THE PRECIPITATION OF ORES. 
One of the most significant facts in regard to the occurrence of ore 
bodies is that they are generally found at the intersection of two faults, one 
of these faults usually dipping steeply, while the other is much flatter. If, 
as we have supposed, the solutions which brought the minerals were ascend- 
ing hot springs, we may further suppose that these springs rose along the 
more steeply dipping or nearly vertical faults. If this is the case, the 
metallic sulphides were not deposited to any extent except at the intersec- 
tion of these steep channels with others which lay flatter; but at such 
intersections there was some. strong motive for precipitation, so that contin- 
uous and rich ore shoots were formed. The flatter-lying fault must also 
have been the channel of solutions, and the explanation is offered that by 
the mingling of solutions which had previously flowed along different chan- 
nels the precipitation of metallic sulphides was brought about. It is not 
possible to decide without more careful study than has been made what the 
exact chemical processes may have been which brought about this result. 
