408 THE GEOLOGY OF MINNESOTA. 



[Mesolite and thomsonite. 



rather high double refraction of thomsonsite (about 0.028). Consequently in an 

 ordinary section of the former the fibres are all uncolored in whatever position they 

 are cut and in some very thin sections they are almost isotropic between the uicols, 

 at least invisible. In thomsonite, however, the fibres show variations of color, and 

 as they are cut more and more obliquely their colors rise until, in a section nearly 

 or quite transverse to their length, the colors are red, blue or green. The color is 

 green in such a section when it is 0.08 millimeter in thickness, and blue or red when 

 slightly less than 0.08 millimeter. 



The variations of light and darkness in a mesolite section or in a lot of fibres 

 mounted in balsam by themselves, are quite small. This is owing to the fact that 

 the optic angle being large and the optic plane perpendicular to the fibres (or 

 sensibly perpendicular) , the sides of the fibres always present n v or n e (or some 

 intermediate pole) in the field of the microscope, and the amount of light capable of 

 passing parallel to >\ is nearly the same as parallel to ,,. It is not so with thomsonite. 

 The optic plane, situated transverse to the fibres as in mesolite, causes some of the 

 fibres to show .,, and n s in the same manner as in mesolite. But in thomsonite the 

 optic angle is small, and the sections perpendicular to the acute bisectrix n g are much 

 darker than those perpendicular to the obtuse ,,. In a longitudinal section, in the midst 

 of a lot of dark fibres (,) may run parallel a lot of brightly illuminated ones (n e ), 

 the contrast being very striking. It is only necessary to apply the convergent lens 

 when it is quickly seen that they only differ in being perpendicular to different axes 

 of elasticity. Such light fibres are scattered, in some cases, amongst the dark ones, 

 and, in finer and finer lines which also become shorter in the finer portions of the 

 slide, they make a network of light and dark fibres, sometimes parallel the one to 

 the other, and sometimes at right angles. These differences are visible in No. 535A(b) 

 above enumerated. 



It will be shown (No. 625B) that in the amygdaloids containing these two 

 zeolites they are sometimes in the same vacuole, the thomsonite being pierced by 

 the needles of mesolite, and somewhat later in date than the mesolite. It appears, 

 by this series of sections that in many cases the two were generated simultaneously. 

 This is shown by some of the bands of the handsome mesolite pebbles. In thin 

 section some of these bands are wholly of white mesolite, and uniformly they 

 are marked by the low tints of the gray of the first order, but, toward the exterior 

 of the band, suddenly begin to appear bright spicules which pierce the darkness of 

 the mesolite. These are thomsonsite fibres cut so as to present the obtuse bisec- 

 trix ,, perpendicular in the microscope. Those which are cut so as to present n s are 

 so nearly dark that they cannot be distinguished from the surrounding fibres of 

 mesolite. Nor indeed can it be stated that the next succeeding band consists wholly 



