128 



ANNALS NEW YORE ACADEMY OF SCIENCES 



Hand specimen 70 consists of a crystallized mass of secondary minerals, 

 among which natrolite, heulandite and datolite are prominent. The slide 

 shows at one point a group of rather large albite crystals within the 

 datolite, evincing much corrosion of outline. 



In 133 and 134, the hand specimen shows large, apparently clear crys- 

 tals of albite, imbedded in beautifully crystallized pectolite. Under the 

 microscope, the albite appears quite turbid in places, and the outlines 

 show evident encroachment of pectolite fibres (fig. 5). In 136, albite and 

 natrolite appear as the filling of a shear vein in dense trap. The manner 

 in which natrolite is replacing albite is sketched in fig. 7. It is evident 

 that the orientation of natrolite has been strongly influenced by the direc- 

 tion of elongation of the albite crystals which they are replacing. In 127 



A^ot^»/t'te 







'/•U, 



TM 





4 



\ \\'\» y wv* f--/' I ' I'll' 'ii 



A/l^/tc 



Fig. 7. Replacement of albite by natrolite. X 35. Slide 13G. 



(fig. 3), the relations are similar, but in this case an intermediate stage 

 is shown, for little groups of prehnite crystals are perched upon the 

 partly corroded terminal faces of the albite. The prehnite, however, did 

 not attain a strong development, and both it and all)ite are giving way to 

 natrolite. It appears probable that, conditions being suitable for the for- 

 mation of natrolite, albite would yield to its advance with great facility 

 on account of the similarity in the composition of the two minerals (al- 

 bite = NagO, AI2O3, eSiO, ; natrolite = NaaO, AI2O3, 3SiO, + 2H„0) . 



In 136, numerous remnants of albite are buried in natrolite fibres, and 

 the natrolite appears to have always assumed a position parallel to the 

 elongation of the albite. Slides 56 and 137 resemble 136 as being minute 

 veins in zones of vertical shearing, which produced sharply defined cracks 

 in quarry walls. In 56, the filling of the veinlet appeared to the naked 



