420 THE CEYSTAL PALLS IRON BEAEING DISTEIOT. 



of which thus holds a great number of hematite plates, aj^pear macroscopic- 

 ally as the little jasper dots already described. Chlorite and apatite are 

 also often embedded in the secondary quartz g-rains, the former in thin 

 plates and the latter in small hexagonal prisms. Epidote is quite common 

 in small irregular areas intercalated between the quartz grains or in the 

 magnetite bands. 



Many of the slides contain a small amount of rhombohedral carbonate, 

 nuich if not all of which is calcite. It occurs chiefly in the quartz bands, 

 in irregular grains which interlock with the secondary quartz grains, and, 

 like them, inclose little crystals of magnetite and hematite. Specimens the 

 slides from which contain carbonates effervesce freely in scattered spots 

 with cold dilute acid. Most of the carbonates are clear white under the 

 microscope, and are evidently calcite. Sometimes, however, the carbonate 

 areas have a very light-brown tint, and are paiiiall}^ surrounded with a 

 limonite border and penetrated by brownish filaments along the cleavages. 

 In such cases it is difficult to decide whether they are calcite stained with 

 limonite, or siderite partially oxidized to limonite. However, if jjart of 

 these areas are siderite it is nevertheless certain that the small magnetite 

 and hematite crystals which they inclose have not been derived from them. 

 These little crystals are inclosed in the carbonates just as they are in the 

 adjoining grains of secondary quartz, while the alteration of the siderite, if 

 it is siderite, is to limonite. Carbonates also occur with tremolite, quartz, 

 chalcedony, epidote, and hematite in the numerous thread-like veins which 

 "traverse some of the thin sections. 



The feldspars have been found in only a few thin sections, as well- 

 scattered but minute angular grains of microcline and plagioclase. Many 

 slides, however, contain areas of matted sericite and quartz which probably 

 represent original grains of feldspar. 



Rutile and tourmaline are also occasionally inclosed with the iron ores 

 in the grains of secondary quartz. Small roundish areas of titanite, prob- 

 ably detrital, occur very sparingly in a few of the thin sections. 



The most interesting features of the thin sections are certain very 

 distinct structural arrangements of the quartz and iron ores. In almost 

 every slide, in ordinary polarized light (with the analyzer out), the minute 

 interpositions of the iron ores are seen not to be equally distributed thi'ough- 

 out the background, but to be concentrated in round or oval areas, never 



