THOMSONITE OF TYPES IICL, III, AND Ilia 467 



appear tp occur together, but they bear the same relationships to the older 

 and younger minerals, and are closely connected by intermediate stages; 

 A careful investigation of individual leaves of II and of Ila, in parallel 

 and convergent polarized light, revealed the same characteristics as are 

 described under type I, with this exception : that the optical axial angle 

 is considerably greater in leaves of types II and Ila than in type I. 

 Means for the determination of the optical axial angle were not at the 

 disposal of the writer. 



TYPE III 



Thomsonite having this habit occurs commonly filling cavities nearly 

 or quite full. It forms solid masses formed of radiating fairly coarse 

 white blades that may reach a length of two or more inches. Where the 

 cavity is not complete^* filled, these radiating aggregates assume hemi- 

 spherical shaped forms, measuring from one to three inches across. They 

 never terminate smoothly, but the fibers may gradually taper out into 

 fine hairs resembling those of mesolite, or more commonly they may 

 become covered with the rough bladed aggregates forming type Ilia. 

 As against the older generations of thomsonite, this variety is quite dis- 

 tinct, although it may merge into mesolite varieties by insensible grada- 

 tions. Figure 2 of plate 44 illustrates a solid amygdule composed of 

 thomsonite of this character. It is deposited upon a coating of thom- 

 sonite of the first type, and this upon chabazite. In figure 2 of plate 45 

 we have a section through one of these radiated aggregates which rests 

 upon thomsonite of the first type. The extremities of the radiating blades 

 extend into delicate separate fibers that resemble the fine hair-like growth 

 to be described as mesolite. Some of these fibers penetrate crystals of 

 analcite that do not show plainly in the figure. In this case therefore 

 the thomsonite is older than the analcite. This, however, is not always 

 the case, for thomsonite of this habit has been observed resting upon 

 analcite as well as on chabazite and on thomsonite of types I and II. 



TYPE Ilia 



The mineral is composed of thin rectangular leaves, confusedly inter- 

 laced, so as to form a very rough porous aggregate that bears a resem- 

 blance to grated cocoanut. Almost always this cocoanut-like mass is to 

 be found on the surface of hemispherical aggregates of the type just de- 

 scribed. Still thomsonite of type III does not always pass into this 

 variety, and Ilia has been observed occasionally quite unconnected with 

 III. Whether it really forms a generation by itself may be doubted. 

 Figure 1 of plate 45 gives a fairly good idea of this habit. 



