DESCRIPTION OF THE IRON ORES. 67 
ORE DEPOSITS IN ANDESITE. 
The deposits within the andesite appear at the surface in long, 
narrow bands, ranging from 20 feet down to less than 1 foot in width, 
and usually standing from a few feet to 20 or 30 feet above the adja- 
cent andesite. (PI. XVIII, A.) These are true veins or fissure 
deposits. The fissures which they fill are usually somewhat curved, 
tapered at one or both ends, are almost invariably branching, and are 
accompanied by subsidiary parallel fissures. (See PL III.) Their 
orientation is diverse; in general they follow the directions of the 
adjacent jointing and faulting. 
ORE DEPOSITS AT ANDESITE-LIMESTONE CONTACT. 
The larger and more numerous deposits are along the andesite- 
limestone contact. (Pis. Ill, V, and XVII, A.) As exposed in the 
erosion surfaces they are commonly lens shaped, with their longer 
diameters parallel to the contours of the hills, but from this there are 
important variations toward irregular polygonal shapes, due partly 
to faulting and partly to the variation in the angle between the ero- 
sion surface and the plane of the andesite-limestone contact which 
the ores follow. The deposits at the contact have as a hanging wall 
either the fresh limestone or the silicated phase characteristic of the 
contact with the andesite. The ore protrudes irregularly into the 
limestone in large and small masses and veins. Small masses of the 
ore, measuring from a few inches to a few feet, may be seen entirely 
wit hin the limestone, and in turn fragments of limestone are found in 
the ore. Along fault planes the limestone is brecciated and cemented 
by ore. Notwithstanding this local irregularity, measured by inches 
and a few feet, the contacts on a large scale are usually even and con- 
tinuous. The dip of the contact of the ore and the hanging wall is 
almost invariably steeper than the dip of the bedding of limestone; 
that is, almost vertical but with a slight dip away from the andesite, 
so far as can be determined from the sections exposed. 
The foot wall of the ore is principally andesite, but at main local- 
ities the ore is separated from the andesite by a thin layer of the sili- 
cated contact phase of the limestone or the sandy basal phase of the 
limestone. The contact of the ore with the foot-wall andesite as a 
whole is considerably more regular than that with the hanging-wall 
limestone. There is less interpenetration of the two masses, vet 
occasional fragments of andesite protrude into the ore or are entirely 
surrounded by it, and andesite breccias with ore cement are not un- 
common along faults. Andesite dikes or offshoots are rare in the 
ores and limestone near the contacts, but are known in one locality 
east of Iron Mountain. The andesite near the contact is altered to 
