456 THE GEOLOGY OF MINNESOTA. 



[Mesolite. 



geometric crystalline forms. They lie in and also cut obliquely or even perpendic- 

 ularly across the mass of the finer fibres. Their cross sections are quadrilateral and 

 sometimes rectangular, in proportion as they are cut more or less obliquely. The fine 

 opaque fibres are so densely compact that their crystalline forms cannot be made 

 out. These both have positive elongation. Between crossed nicols the fine fibres 

 are-the only illuminated portion of the section, as enough light gets through to 

 render them distinctly separable from the transparent fibres which in that condition 

 are wholly dark. In natural light, however, the larger transparent fibi-es are the 

 more illuminated. The mass of finer fibres, which are sub-opaque, are seen to be 

 charged with minute dark impurities, and these are sometimes so large as to 

 indicate that they are iron-stained, and are probably due to partial decay. The larger, 

 transparent fibres are free from such impurities, but show a scant cleavage nearly or 

 or quite perpendicular to the elongation. The specific gravity of this substance is 2.26. 







Micro-chemical test (Boricky) gives evidence of the presence of considerable 

 soda, along with lime, or lime and magnesia. The rods (hexagonal) of fluosilicate 

 of soda are innumerable, but so fine that they are likely to be overlooked in the 

 presence of the conspicuous monoclinic crystalliths of the alkaline earth. They are 

 brought out to view by removing the upper nicol and lowering the lower nicol, 

 when they seem to equal, if not exceed, in total amount, the crystalliths of lime. 

 This prevalence of soda indicates a zeolite of the natrolite group, in which Dana has 

 only three recognized species, viz.: 



Natrolite (or mesotype). 



Scolescite. 



Mesolite. 



These are all subject to ready alteration and to partial or total loss of the power 

 to polarize light. 



1. Natrolite is orthorhombic. 



2. Scolescite is monoclinic. 



3. Mesolite is monoclinic (or triclinic, Descloizeaux). 



The first contains soda only as a base; the second contains lime only, and the 

 third has lime and soda. The Boricky test determines this of these three, to be the 

 last, viz.: mesolite. 



It seems quite possible, if not probable, tlftit two zeolitic minerals of the natrolite 

 group of Dana are here concerned, viz.: mesolite the finest and somewhat altered, 

 and mesotype the clear and transparent, the latter being older than the former. Both 

 kinds of fibres have a close striation or parallel cleavage, and a distant fissuring 

 perpendicular to their elongation. 



Age. Manitou. 



Remark. These numbers (625A and 625B) also cover other zeolitic minerals. 



N. H. W. 



