4 PKOCEEDINGS OF THE NATIONAL MUSEUM. vol. 64 



microscopic examination these are found to be minute spherulitic or 

 spindle-shaped grains of quarts with radial structure. Koch selected 

 what he thought was a very pure material for analysis and estimated, 

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

 analyzed material had contained about 33 per cent of quartz. Schal- 

 ler's recalculation shows that about 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, al- 

 though 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 in the cavity. Within 

 this there is usually a thin layer of chalcedony upon which the mor- 

 denite was deposited, filling the interior of the cavity with loosely ag- 

 gregated fibers. A typical specimen of this sort, although one of the 

 smallest received, is that shown in plate 1, upper left. Many of the 

 cavities have a thicker crust which is found upon microscopic exami- 

 nation to consist of chalcedony including mordenite fibers which have 

 the same attitude as those which occur free 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 2, lower, being an example, while 

 in extreme cases, noted only in the smaller masses, the entire content 

 of the cavity has been saturated with chacedony, or, more rarely, 

 granular quartz. One striking specimen illustrated in plate 3, lower, 

 has silky bundles of short fibers of mordenite attached to masses of 

 small quartz spherulites which project from platy (argentine) calcite. 

 Analcite occurs as flattened crystals between the calcite plates. Both 

 this spheruhtic quartz and the calcite are probably younger than the 

 mordenite and the calcite has apparently, in part, been dissolved 

 away. The microscope shows the mordenite fibers of this specimen 

 to be individually coated with a thin film of calcite. In other speci- 

 mens the fibers were similarly incrusted with quartz. A part of the 

 light cottony vaiiety contained many of the small radial grains of 

 quartz but was free from the incrustations. This was separated from 

 its included quartz grains by alternate rubbing between hardwood 

 boards and screening, the quartz grains when freed from the fiber, 

 passing through the meshes of the sieve while the tufts of mordenite 

 remained behind. This process was continued until no more gritty 

 particles could be felt in the sample when it was examined optically 

 and found to be pure and free from quartz. Upon analysis this sam- 

 ple gave the following results and ratios. 



