400 H. B. PATTON — THOMSONITE, MESOLITE, AND CHABAZITE 



Thomsonite of this habit is very likely to be the first mineral deposited 

 on the roof and sides of the cavities, and, where there is no bedded floor 

 composed of mixed zeolites, this is also true of the bottom of the cavities. 

 Usually, however, it is laid down on a thin coating of chabazite. 



TYPE II 



Thomsonite of this type never forms compact aggregates. The indi- 

 vidual crystals are usually rectangular tablets of small size (1 millimeter 

 or less), but still they are relatively much larger than the individual 

 leaves of t} r pe I. These little tablets bunch themselves together into 

 delicate, snow white, prismatic aggregates that frequently taper to a point 

 or branch into smaller offshoots. The whole interior of a cavity may be 

 thickly covered with a growth of the delicate tapering and branching 

 prisms such as is shown in figure 2 of plate 43. Sometimes these com- 

 plex prisms are so fragile that a slight pressure of the fingers will crush 

 them. More frequently they are firm enough to stand considerable 

 pressure, such as would be produced by wrapping in cotton and tissue 

 paper. The complex prisms usually attain a length of a quarter to half 

 an inch. Not infrequently thomsonite of this type forms more or less 

 parallel continuous prismatic aggregates a quarter to half an inch thick, 

 completely coating a cavity. In this case the appearance described above 

 is produced by the continued grow r th of some of the complex prisms form- 

 ing the base. Occasionally one may notice this variety of thomsonite 

 directly coating the surface of the cavity or lying on a thin film of 

 chabazite. Nearly always, however, a close examination will disclose an 

 underlying layer of thomsonite of the first type. The aggregates com- 

 posing this second growth do not, however, continue the growth of the 

 first generation, but rest quite indifferently on this as a base. 



TYPE I la 



No sharply defined line can be drawn between this and type II. In 

 its best developed form it consists of similar complex, prismatic aggre- 

 gates diverging regularly and symmetrical!}' from common points so as 

 to form beautiful delicate hemispherical bunches. At the same time the 

 delicate prisms become longer and straighter and taper out into hair- 

 like forms, and thus appear to pass insensibly into the mineral mesolite. 

 These hemispherical balls may sometimes coalesce, but usually they lie 

 quite distinct and beautifully regular. Figure 1 of plate 44 presents an 

 excellent likeness of this habit where the prisms are fairly coarse and 

 firm. It lies here on a continuous layer of thomsonite of the first type. 

 Apparently types II and Ila belong to the same generation. They do not 



