092 GEOLOGY AND TOPOGRAPHY OF THE LEAD REGION. 



the zinc ore below the water level is usually blende (sphalerite) with but little admix- 

 ture of the carbonate. As the level of the water in the ground becomes gradually lower, 

 and it is a well known fact that it does, the atmosphere, together with surface water 

 charged with carbonic acid, is permitted to act upon the blende, and a transformation 

 from the sulphuret to the carbonate is the result. 



The association of calcite with other minerals is such as to indicate that it must have 

 been formed in crystals during at least two different periods. Stalactites of recent ori- 

 gin are found in the mines, which on being fractured show a distinct crystalline struc- 

 ture, and large planes of cleavage. 



The following is a list of the minerals known to occur in the Lead region, arranged 

 according to the system adopted by Prof. Dana, in his Mineralogy : 



Sulphur. Native sulphur is found but seldom in the Lead region; its presence is 

 usually due to the decomposition of iron pyrites. It is usually found in a pulverulent 

 form. Some pieces weighing as much as an ounce were seen in a cabinet at Hazel 

 Green, which are said to have been obtained from a small sheet in some of the Bun- 

 come mines. It is said to be not uncommon in this vicinity. Other localities where it 

 is found are, Mineral Point and the Crow Branch diggings. 



Bornite. Variegated or Purple Copper ore. Composition: copper 62.5; iron 13.8; 

 sulphur 23.7. This is quite a rare mineral; a few pieces have been found in the copper 

 diggings near Mineral Point; it has never been found here crystallized, but always mas- 

 sive and in small pieces. 



Galenite. Composition: lead 86.6; sulphur 13.4. This is the only ore of lead found 

 in sufficient quantities to be of economic value; it is universally known in the Lead re- 

 gion as "mineral." It frequently occurs in distinct crystals, either as a cube or some 

 modification of it. Octahedral crystals are quite rare, but are occasionally found, espe- 

 cially in the carbonaceous shale of the southern part of the region. Usually, however, 

 galenite occurs massive, with a very distinct cleavage. Freshly broken surfaces have 

 always a bright steel color, which speedily tarnishes on exposure, to the air. 



Sphalerite. Blende or Black-jack. Composition: zinc, 67; sulphur, 33. This is 

 one of 'the most abundant minerals in the Lead region, besides being of great economic 

 value as an ore of zinc. It is almost invariably found as an associate vein-mineral in the 

 horizontal deposits of Lead ore. It is usually found massive and compact, of a dark 

 brown or black color, due to a small portion of iron contained in it, and more or less 

 mixed with galenite. The Lead region has never afforded a perfect crystal of blende, 

 although many specimens are found with small and imperfect crystalline faces. The 

 fractured surfaces of such specimens usually have a resinous luster. 



Pyrite. Composition: iron, 46.7; sulphur, 53.3. This is the most common vein- 

 mineral found in the mines; it is universally met with in veins, lodes or other deposits 

 of ore, and in many cases impregnates the rock when all other metals are absent. In 

 crevices it frequently appears to have been the first mineral deposited. It is usually 

 found massive, although handsome crystallized specimens are frequently obtained from 

 the mines. In crystals it usually assumes some modification of the cube, the octahedron 

 being quite frequent. It also occurs in radiated and reniform masses. It has never yet 

 been considered of any economic value in the Lead region, and as it is so much mixed 

 with rock, it is doubtful if it could be profitably separated, except by the natural process 

 of disintegration, to which some varieties are liable when exposed to the air. The Crow 

 Branch diggings and the Linden mines afford large quantities and good specimens of 

 this mineral. 



Marcasite. Composition: iron, 46.7; sulphur, 53.3, or same as pyrite. The difference 

 between this and the preceding is but slight, and chiefly due to crystalline structure; 

 the former belonging to the monometric and the latter to the trimetric system. It is 

 somewhat lighter colored than pyrite, and decomposes more readily in the air. It is 



