PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 239 



Stilbite. Diabase. Heulniulitr. rnlrite.] 



No. 172A. STILBITE. 



From No. 172. 



Ref. Annual Report, ix, page 46. 



Meg. A slickensided sheet of stilbite from a vein. The crystals stand out from 

 the sides of the sheet, and along a cavity in the centre of the sheet show rough ter- 

 minations. A little thalite and some small fragments of the adjacent rock are 

 included in the specimen. 



No section. 



Age. Manitou. u. s. G. 



No. 173. DIABASE. 



Northeast corner sec. 28, T. 59-4 W. In a little stony bay facing northeast. This bay is partly shut in by 

 a projecting trap point running northeast, from which this number is obtained. It is an amygdaloidal trap con- 

 taining heulandite, thalite, calcite, with some laumontite in amygdules and in nests and joints. The heulandite 

 occupies the larger cavities, or lines them, the thalite being as filling to amygdules or in geodes of heulandite. 

 The rock itself is roughly bedded, and dips toward the lake at an angle of about 10. 



Ref. Annual Report, ix, page 46. 



Mrcj. A dark, brownish-gray, fine-grained, diabasic rock. Small, white, appar- 

 ently pseudamygdaloidal areas are common. These contain calcite, thalite and 

 heulandite ('?), and the rock is more or less penetrated by very minute areas of these 

 minerals. 



Mic. The section shows a fine-grained diabase, similar in structure and compo- 

 sition to No. 172. There are a few plagioclases considerably larger than the usual 

 feldspar laths, and these give a semi-porphyritic aspect to the section. In several 

 areas is a very finely fibrous mineral, the elongation being in a negative direction. 

 Commonly, in polarized light, there appear fine, dark, curving bands, on each side 

 of which are radiating fibres. Occasionally these dark bands are double, with a 

 narrow light band between them which extinguishes with the fibres on each side of 

 the band. The double refraction is rather low, and the fibres rarely show colors 

 higher than bright gray of the first order. The mineral is perhaps chalcedony. The 

 feldspar is often much altered and is sometimes partially replaced by this fibrous 

 mineral. 



Running partly through the section is a vein of a very cloudy mineral with a 

 low index of refraction and very low double refraction. The cloudiness is due, at 

 least in part, to minute inclusions which are arranged with their long axes approxi- 

 mately parallel to an axis of elasticity, which is greater than the axis at right angles 

 to these inclusions. This mineral is perhaps stilbite. 



One section. 



Af/e. Manitou. u. s. G. 



No. 173A. HEULANDITE AND CALCITE. 



From No. 173. A mass of crystals of heulandite two inches in thickness and about four inches wide. 

 Ref. Annual Report, ix, page 46. 



