Mesolite and thomsonite.] 



PBTROGRAPHIC GEOLOGY AND DESCRIPTIONS. 



407 



places on the north shore of lake Superior, from the same series of diabase sheets, 

 show plainly the close intermixture of two zeolites, mesolite and thomsonite, the 

 former being more abundant than the latter. 



fibre of Thomaonite. 

 A/m NT* 



filice of J^fesolite. 



Mm. 



fifm 



Negative, 



Ma.*k*d. difference of il Rumination, on. 



2 V Is very large. 

 Nearly equal illumination, on Np and. 



an.ee /Vgt. 



FIG. 23. DIAGNOSTIC OF THOMSONITE AND MESOLITE. 



The concentric growth, shown by the eyelets seen on the exterior of the beach- 

 worn pebbles, is evinced in the thin section by concentric bands of varying colors. 

 These bands vary in composition as well as in fineness of fibre, sometimes fading 

 gradually into each other, and sometimes changing rather abruptly. 



Of the specimens chosen the following notes were taken prior to the making of the sections: 



(a) No. 535. Amygdaloidal rock, containing small zeolitic spherules supposed to be thomsonite. 



(b) No. 535A. Coarsely fibrous zeolite, possibly stilbite, from the rock. 



(c) No. 535A. Supposed thomsonite, two sections, from the rock. 



(d) No. 535A. Supposed thomsonite. 



(e) No. 535B. Supposed thomsonite, from the beach, with some of the rock, two sections. 



(f) No. 535B. Supposed lintonite, from the beach, with some of the rock. 



(g) No. 535B. Supposed thomsonite, from the beach, pyramidal form, very common. 



Of these the following results were reached after petrographic and micro-chemical investigation, viz.: 



(a) The amygdaloidal rock (a) contains only thomsonite as zeolitic filling. 



(b) Composed of thomsonite. 



(c) Composed of mesolite banded with minute fibres of thomsonite; specimens are quite similar. 



(d) Composed of mesolite banded with minute fibres of thomsonite. 



(e) Mesolite, with spheruliths in the rock and in the mesolite surrounded by another undetermined min- 

 eral which seems to be gonnardite, as described by Lacroix (Mine>alogie de France, vol. ii, page 279), and also 

 to resemble lintonite. 



(f ) Is not homogeneous, but consists largely of some amphibole, hardly dichroic, of zoisite, sphene and 

 of some other undetermined fine-grained substances. 



(g) Proves to be mesolite. 



Of these numbers c, d and g represent the variegated and banded pebbles which 

 are credited, in recent publications, to Grand Marais, as thomsonite, particularly c and 

 d; while the pebbles that are actually of thomsonite are usually not chosen for their 

 beauty and are coarse-grained and non-banded. 



Thomsonite and mesolite have not only similar chemical composition, but 

 similar optical properties. Yet in two important particulars their characters are so 

 different, under the microscope, that they are easily distinguished. In general also 

 it can be said that the fibres of thomsonite are coarse and irregular compared to 

 those of mesolite, which are fine, rigid and straight. The distinguishing difference, 

 however, is in the low double refraction of mesolite (about 0.012) compared with the 



