462 THE GEOLOGY OP MINNESOTA. 



[Heulandite. 



disseminated generally through the mass, especially coating the amygdules and 

 sometimes filling them. 

 No section. 



Age. Manitou. N. H. w. 



No. 634A. HEULANDITE, ETC. 



Amygdules picked from No. 634. They consist of heulandite, agate, stilbite, calcite and thalite(?) 

 Ref. Annual Report, x, pages 63, 64; Annual Report, xi, pages 171, 181. 



Meg. These vary in size from that of a mustard seed to masses two or three 

 inches across. They are usually coated with red. 



Heulandite, which, though resembling stilbite, is distinguished from it readily 

 by the interference figure, which is that of a bisectrix, which its cleavage plates 

 afford in convergent light, is the most abundant of these amygdaloidal minerals. 

 Its cleavages give a bright, even silvery, lustre, and they are so small sometimes that 

 they can only be discerned with a magnifier. This mineral fills some large openings 

 in the rock. It also becomes granular and then is reddened by ferric oxide, and in 

 this condition it gives color to large surfaces, lining veins, coating the globular 

 bodies that are detached from the rock and at the same time coloring more or less 

 the molds from which they are loosened. 



Stilbite is in large masses and also becomes red, passing through a light flesh-red 

 tint. It also becomes confusedly lamello-fibrous, and fibrous with radiated structure, 

 when it presents the characters of puflerite, and in this form it also coats the outer 

 surfaces of the detached amygdules, except that the fibrous structure is then not so 

 perfect, passing, probably in association with more or less of heulandite, to finely 

 granular. A thin section cut so as to cross both the lamellar and the reddened and 

 fibrous portions indicates that the whole is still in more or less distinct lamellae, 

 which always show a negative elongation, but lose their optic characters in places 

 in consequence of the prevalence of much Jieinatite. 



The stilbite is in large masses. Sometimes its undulating cleavage surfaces 

 give a colored iridescence, which is doubtless owing to the action of the highest 

 double refraction, which takes place perpendicular to the cleavages. This phenom- 

 enon is not observed in heulandite. 



The siliceous secretions are in the forms of banded agate and coatings. A 

 banded, agate-like layer frequently was the first deposit on the walls of the cavities, 

 particularly the larger ones. Next within this is frequently stilbite, but between 

 the banded siliceous layer and the stilbite is also frequently a band of massive or 

 granular quartz. The globular agates are finely and exquisitely striped. 



In a thin section, which is mainly red, a greenish thalite(?) is permeated and 

 pierced by slender spicules of red hematite (?) 



Aye. Manitou. 



