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



may be justified in stating the contrast in still stronger terms, inasmuch 

 as it is difficult to realize that the several specimens really represent the 

 same mineral, irrespective of their common origin. This great diversity 

 is undoubtedly due to the fact that the several varieties have been formed 

 under varying conditions. It is, indeed, possible and easy to recognize 

 several generations of thomsonite, each generation having its own habit. 

 As a matter of fact, two such generations were recognized and described 

 by Cross and Hillebrand. Their description of the first generation is 

 very exact, and specimens of this type may easily be identified from such 

 description. It is not so easy, however, to identify their second genera- 

 tion with any individuals of the later ones described in the following 

 pages. Without the aid of photographs or other illustrations, it is no 

 easy matter to convey an accurate conception of minerals whose forms 

 are not clearly cut. It is to be hoped, therefore, that the reproductions 

 of carefully made photographs accompanying this paper will give some- 

 thing like an accurate idea, impossible through the printed page alone, 

 of the occurrences under discussion. 



Altogether three distinct types may be distinguished, representing 

 successively later generations. In at least two cases one or two sub- 

 types are recognized and are designated by the letter a. It is not, how- 

 ever, always possible clearly to distinguish these subtypes, as the}'' pass 

 into each other by many intermediate stages. In case of the three main 

 types, designated as I, II, and III, there is ordinarily no difficulty in 

 making the proper distinction. This may be done by the peculiarities 

 of development, as well as by the association with the other zeolites and 

 with each other. 



TYPE I 



This represents the ordinary habit of thomsonite as it occurs in many 

 places on both Table mountains, and is very characteristic of the locality. 

 The following description of this habit, taken from the paper by Cross 

 and Hillebrand, fits the occurrence admiriibly : 



"The mineral scours in very minute rectangular blades, which are placed upon 

 each other like the leaves of a closed fan, and the very compact combinations of 

 such aggregates are usually arranged in a more or less distinct radiate manner. 

 Sometimes spherical forms result, in other cases columns, by radiation from an 

 axis, or, less frequently, walls, the blades standing at right angles to the central 

 plane. . . . When a large surface of chabazite has been completely coated by 

 the more or less radiate aggregates of thomsonite, forming an undulating surface, 

 the whole has a most delicate silken luster, while that on a fractured surface of a 

 spherical mass is more satin-like." 



