DESCRIPTION OF ROCKS AND ANALYSES 21 
vestigations of Rinne! upon change of optical properties on zeolites 
by heating state lowering of the birefracting power (y—e) on thom- 
sonite. — 
On the free ends the radiating groups of the thomsonite are often — 
covered with small radiating sheaves of a weathered yellowish mi- 
_mneral of a much stronger birefracting power on the cleavage flakes, 
but showing emergence of the optic normal (@) and an extinction 
angle (c:y) of about 18°, all that on the extremely few parts of the 
mineral, which are not changed into isotropic substance. The shape 
of the mineral and these few optical indications point to altered 
laumontite. 
In the specimens brought home, no primary calcite nor silica 
minerals were detected amongst the amygdale and. vesicle fillings, 
whose order of deposition is: | 
analcite, 
natrolite, 
thomsonite, 
 laumontite, 
an order which denotes an increasing water and lime content during 
the gradual cooling of the rock in the late magmatic and postmag- 
matic (hydrothermal) stage of development, corresponding to a de- 
crease in soda, an inverted sequence as compared with the early and 
true magmatic stage. — 
In thin sections the rock exposes an intersertal structure with 
phenocrysts of plagioclase feldspar and olivine only slightly exceeding 
the groundmass minerals by dimensions. The phenocrysts of plagio- 
clase are slightly twinned and distinctly zoned with a core corres- 
ponding to Ab,,An,, and an envelop of 45 °/o An; the outer contour 
of the plagioclase grasps in the groundmass by slight dentification. 
The lath-shaped groundmass-plagioclases are simple albite-twins with 
uneven border and an anorthite-content of 40--45 °/o and lower; 
1 F. Rinne, Sitz.-Ber. Berliner Akademie 46. 1890. p. 1174. 
