THE MINERALS OF IDAHO 347 



compact porcellaneous materials. Most abundant and conspicuous 

 are the large geodal masses of fine cottony material which reach a 

 diameter of 30 cm. or more. These vary from round to irregular 

 in cross section and consist, usually, of a thin shell of chalcedonic 

 material filled with the light tufted fibers of the mordenite. Although 

 this cottony material appears entirely homogeneous to the unaided 

 eye it all contains gritty particles which can be felt when it is crushed, 

 and upon microscopic examination these are found to be minute 

 spherulitic or spindle-shaped grains of quartz with radial structure. 

 Koch selected what he thought was very pure material and estimated, 

 after the analysis had shown an abnormal amount of silica, that the 

 analyzed material had contained about 33 per cent of quartz. 

 Schaller's recalculation showed that 40 per cent would be nearer the 

 truth. The small quartz spherulites are pinned through by the mor- 

 denite fibers and have clearly developed subsequent to the mordenite. 

 None of the fine cottony material was found to be free from quartz, 

 although the compact varieties are frequently free from any impurity. 

 Practically all of the geodes of the cottony type have a very thin 

 outer shell of heulandite which forms the first lining of the cavity. 

 Within this there is usually a thin layer of chalcedony upon which 

 the mordenite was deposited, filling the interior of the cavity with 

 loosed aggregated fibers. A typical specimen of this sort, although 

 one of the smallest seen, is that illustrated in Plate 10, upper left. 

 Many of the cavities have a thicker crust which is found upon micro- 

 scopic examination to consist of chalcedony, including mordenite 

 fibers, which have the same attitude as those which occur in the 

 middle portion of the cavity. This chalcedony appears to be later 

 than the mordenite and to have formed a crust by saturating the pore 

 space of the loose textured mordenite. In some geodes of moderate 

 size only a small area remains in the center where the mordenite is 

 not saturated with silica, the specimen shown in Plate 11, lower, 

 being an example while in extreme cases, noted only in the smaller 

 masses, the entire content of the cavity has been saturated with 

 chalcedony or, more rarely, granular quartz. One striking specimen, 

 illustrated in Plate 12, lower, has silky bundles of short fibers of mor- 

 denite attached to masses of small quartz spherulites which project 

 from platy (argentine) calcite. Analcite occurs as flattened crystals 

 between the calcite plates (pi. 12, right). Both this spherulitic 

 quartz and the calcite are probably younger than the mordenite and 

 the calcite has apparently, in part, been dissolved away. The micro- 

 scope shows the mordenite fibers of this specimen to be individually 

 coated with thin films of calcite. In other specimens the fibers are 

 similarly incrusted with quartz. 



A part of the light cottony material which contained many of the 

 small radial quartz grains w r as carefully purified by a special process 



