PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 229 



Heulandite. Mesolite.] 



No. 161 A. HEULANDITE. ( Veins.) 



Veins from two to four inches wide, run irregularly in No. 161. Apparently these mark openings in No. 

 1G1 through which corrosive vapors or waters have passed, converting the rock on either side into a finely 

 granular brown mass in which heulandite is the only identifiable mineral. 



Ref. Annual Report, ix, page 42; Annual Report, xiii, pages 100 (No. 161), 103. 



This brown rock was analyzed by Prof. J. A. Dodge with the following result (Annual Report, xiii, 

 page 100): 



SiO 2 50.31 



A1 2 O 3 14.17 



Fe 2 O, 10.96 



FeO 1.09 



CaO 8.44 



MgO 5.86 



K 2 O - 0.46 



Na 2 O 0.90 



Water - 7.63 



Total 99.72 



Aye. Manitou. - N. H. w. 



No. 161B. MESOLITE. (Pebbles.} 



From the beach, supposed to be thomsonite when gathered. Derived from the top of No. 161. 

 Ref. Annual Report, ix, page 42. 



Meg. This is a very hard and tough milky white, finely fibrous mineral, the peb- 

 bles sometimes being more than an inch in diameter. It is not conspicuously orna- 

 mented with the bands and cat's-eyes seen in the typical " thomsonite " of the region. 



Mir. In a section made parallel to the fibrous structure, the fibres are "seen to 

 be very fine and short, interlocking and overlapping in a strong, dense, elongated, 

 feathery network, which between crossed nicols is nearly opaque by reason of the 

 overlapping of the fine fibres, giving only scattered acicular bright points, but 

 amongst which are distributed many more minute light points. These acicular 

 crystals have a negative elongation and show sometimes the terminal forms of 

 monoclinic or triclinic crystals. They extinguish when nearly parallel with the 

 axes of the nicols. They are frequently pointed at both ends, which is distinctly 

 seen in polarized light, the rest of the field being dark. The polarization colors are 

 very low or about that of quartz when viewed in this direction. It is evident that 

 there is a banding, or zonal structure, which crosses the direction of the fibres at 

 right angles. This is apparent on viewing the thin section with the naked eye. 



Another section, made thinner, at right angles to the direction of the fibres, is 

 still slightly fibrous, a few of the fibres apparently running nearly at right angles to 

 the main structure of the mineral, but in general this section is divided into small 

 angular areas which each represent the cross-section of one of the needles. There is not 

 any noticeable difference in the power of double refraction. Gelatinizes with HC1. 

 Fuses rather easily to a white enamel, apparently containing minute bubbles. 



Micro-chemical tests by the Boricky method afford many magnificent crystalliths 

 of lime, with hexagonal rods of soda. The hardness, the color, the structure and the 



