CLASSIFICATION OF ORE DEPOSITS. 57 



dead and decaying, is notable. Its office even in precipitating the 

 gangue minerals in surface reactions we are just beginning to 

 appreciate. Siliceous sinters have been shown by W. H. Weed to 

 be formed around the hot springs of the Yellowstone Park through 

 the agency of algae, and A. Rotbpletz has recently proved that the 

 calcareous oolites around the Great Salt Lake are referable to mi- 

 nute organisms. Many accumulations of iron ores have, with reason, 

 been attributed to the same agency; but for this metal ordinary 

 and common chemical reactions are oftenest applicable. When or- 

 ganic matter decays, sulphurous gases are one of the commonest 

 products, and likewise one of the most vigorous of precipitants. 

 Thus under Example 24, Paragraph 2.06.03, when speaking of the 

 Wisconsin zinc and le^d mines, it will be seen that such an agency 

 from decaying seaweeds has been cited by both Whitney and 

 Chamberlain. When the products of such decomposition become 

 imprisoned in the rocks as oils and gases, their action is unmistak- 

 ably important and is especially available in limestones. Organic 

 matter is a powerful reducing agent as well, and in this way is ca- 

 pable of bringing down metallic compounds. The silver-bearing 

 sandstones of southern Utah are cases in point, as they afford plant 

 impressions now coated with argentite. The purely physical agen- 

 cies cited under (cl) have also an important role. 



1.06.18. Under 2 (a) the uprising solutions may be diverted by 

 porous strata so as to soak through them and become subject to 

 precipitating agents of one kind or another. They furnish the 

 simplest kind of cavities, and starting with these the scheme is de- 

 veloped in a crescendo to the most complicated. The purely chem- 

 ical action of limestone beds, however, seems at times to come into 

 play and to cause precipitation along them. Of all rocks they are 

 the most active chemical reagent. It may be questioned with rea- 

 son as to whether caves or caverns (4), properly so called, have ever 

 formed a resting-place for ores. So many which have been cited as 

 such may with greater reason be referred to shrunken replacements 

 that a doubt hangs over their character. 



1.06.19. Under (5) brecciated beds whose fragments are coat- 

 ed and whose interstices are filled with ore are, with great reason, 

 referred to the collapse from the removal of a supporting layer. In 

 addition to the illustration cited, the red hematite deposits of Dade 

 and Crawford counties, Missouri, have been thought to have had a 

 similar origin. Such phenomena are only to be expected in regions 

 that have long been land. Cracks at the bends of folds may, in 



