GEOLOGY IN NATIONAL MUSEUM MERRILL. 801 



The collection as displayed in a wall case at the west end of the hall 

 is selected to sIioav kinds and geographic distribution. Inasmuch 

 as this water itself presents to the eye no marks of differentiation 

 the samples are displayed in the original package. The collection 

 is accompanied by a map showing distribution in the United States. 



THE GEOLOGY AND MINING INDUSTRY OF LEADVILLE, COLORADO. 



A wall case at the north end of the east range on the first floor has 

 been devoted to this collection, of which the following is a transcript 

 of the label : 



The prevailing and most important ore of Leadville is an argentiferous galena, 

 which below the zone of oxidation is associated with zinc blende and pyrite. 

 The secondary silver-bearing minerals are argentiferous cerussite, or carbonate 

 of lead; cerargyrite, or chloride of silver; the chlorobromide of silver; less 

 frequently chloroiodide of silver; and very rarely native silver. Lead also 

 occurs in secondary forms, as anglesite, or sulphate of lead ; pyromorphite or 

 chlorophosphate ; occasionally as litharge, and rarely as minium. Zinc occurs 

 in the form of calamine, or silicate, while iron occurs as magnetite, hematite, 

 or limonite, oxidation products of pyrite. Gold occurs in the native state in 

 the form of small leaflets. More rarely occur ores of manganese, arsenic, anti- 

 mony, molybdenum, copper, bismuth, and vanadium. By far the most im- 

 portant of the ores occur in the blue-gray dolomitic limestone of the Lower Car- 

 boniferous formation and associated with intrusive bodies of porphyry, either 

 overlying or cutting across it, and which are known as white, or Leadville por- 

 phyry, and gray porphyry. The gangue of the ores is mainly some form of 

 silica (mechanically or chemically combined with hydrous oxides of iron and 

 manganese) and a great variety of hydrous silicates of alumina and barite. 

 The main mass of argentiferous galena or lead ore is found in calcomagnesian 

 beds. The ores containing gold and copper are more frequently found in 

 siliceous beds, in porphyries, or in crystalline rocks. 



The ores are assumed to have been deposited from aqueous solution and 

 originally in the form of sulphides. They are of later origin than the inclosing 

 rock, and were deposited not later than the Cretaceous period. It is believed 

 that the metallic contents of the ores were derived mainly from the neighboring 

 and overlying eruptive rocks. 



The exhibition comprises all the principal types of rocks and ores, 

 together with a relief map of sections of the region, as collected and 

 produced by the U. S. Geological Survey. 



CENSUS OF THE COLLECTIONS, DEPARTMENT OF GEOLOGY. 



The exact number of individual specimens in the department can 

 not be given as may readily be understood on a moment's considera- 

 tion. A rock fragment to-day considered one specimen may to-mor- 

 row be broken into a dozen pieces, and a single handful of small fos- 

 sils may comprise a hundred or thousand individuals. In the exhibi- 

 tion series it is the custom to consider as one specimen the material 

 covered by a single label, though it may weigh from a grain to a ton 

 or more, whether it be a single fossil of pinhead size, or an aggregate 



