116 Review of Hail and Whitney's Geology of Iowa. 
- arrangement, so far as could be made out from the information 
collected. In no part of the lead region are the crevices devel 
on a more extensive scale, or with so much regularity, 
in the Dubuque district. The characteristic form of occurrence 
of the ore is the cave-opening, or expansion of the vertical crevice 
into a cave or chamber, whose walls are sometimes lined witha — 
heavy incrustation of pure galena, but which are more Spee : 
partially filled with clay and loose masses of rock, mixed with 
fragments of ore, derived from the decomposition of the material 
which once filled the opening, or metalliferous portion of the — 
rock. Some of these caves have yielded several millions of 
the following . 
“There is very little evidence that the crevices continue to be 
productive, in the Dubuque district, even as low down as the Blue 
estone ; and it is certain from the study of the whole region, 
underlying this sandstone (the Lower Magnesian), when thisT0® — 
occupies the surface ; but the deposits in that geological posite 
are very few in number, and the ore limited in quantity; We 
have yet to learn of a single instance in which diggings in %® 
rock have been profitable for any length of time. But, ag%% — 
even if the Lower Magnesian were a good mineral-bearing Toc 
there would be little encouragement to continue sinking fom — 
the Galena limestone, through the sandstone, into the underlyi?3 _ 
sandstone; for there is no reason to suppose that a crevice, &*) 
being entirely interrupted in the sandstone, would be resumed ai 
in the limestone below, at a point exactly in the line of directiop 
of the workings above. A miner aac be no more justified # : 
* Report p. 462. 
