PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 227 



Stilbite. Amygdaloid.] 



abundant amygdules of saponite, calcitc, stilbite, ntexolite and heulantUte. While the 

 saponite is usually soft, and coats the other arnygdaloidal minerals, in the case of the 

 mesolite it is intimately intermingled and becomes very hard (lintonite ?) The mass of 

 the rock has also been so permeated by decay that saponite is formed throughout it. 

 A</i>. Manitou. N. H. w. AND u. s. G. 



No. 158A. STILBITE, ETC. 



Same locality as No. 158. 



Compare Nos. 630, etc. 



Kef. Annual Report, is, page 41. 



Meg. The sample is like the amygdaloidal sample of No. 158, and contains one 

 large mass of stilbite in the midst of which are also laumontite and mesolite. The 

 last gelatinizes readily in hydrochloric acid, and fuses easily to a white porcellanous 

 bleb. N. H. w. AND u. s. G. 



No. 159. AMYGDALOID. 



Prom the extreme east end of Little Marais bay, underlying No. 160, which forms the point on the east 

 side of the bay. 



Ref. Annual Report, ix, page 41. 



Outwardly this amygdaloid in the field is associated with fragmental and 

 brecciated material, being probably the upper surface of a lava flow, more or less 

 worked over by atmospheric and aqueous agents. 



Mic. The rock itself is porphyritic with feldspars of small dimensions, and 

 shows many that are microscopic. It does not differ essentially from the basic 

 amygdaloids which prevail along this part of the lake Superior shore, being closely 

 allied to No. 158. 



It contains two zeolites. One of them, having an easy, flat cleavage, affords a 

 lot of thin flakes which, in convergent light, show n e perpendicular to the cleavage. 

 The acute angle of the optic axes seems to be small, as the whole interference figure 

 is visible even without the use of methylene. A measurement by Lacroix's axial 

 goniometer gives 2E=7S 46'. This zeolite is evidently heulttiidite, its appearance 

 also being quite similar, megascopically, to that found in No. 637. 



Sometimes this mineral shows a shaded banding resembling a kind of albite 

 twinning apparent in the thin flakes. When the flakes are thick the interference 

 figure does not form a perfect dark cross at any time, but still exhibits character- 

 istically the loci of the optic axes. 



The other zeolite is hniniontite, having a fibrous, yet occasionally a laminated, 

 structure, the latter parallel to 010, and the axes , perpendicular to the face 010. 

 Extinction makes an angle of 28 with the elongation which is parallel to the edge 

 100:010. 



Two sections examined. 



Age. Manitou. N. H. w. 



