ZOISITE. 81 



cliroism or slight dichroism, by the colors of interference, und, when seen 

 in cross-section, by the angle of extinction. The more highly colored 

 zoisite in prisms bears a superficial resemblance to augite, which need.-- 

 only to be pointed out to avoid confusion. The mineral in the form 

 of small prisms and needles may readily be confounded with apatite. 

 The latter, however, does not give the j-ellow interference tints of zoisite 

 and seldom shows the light-green tint in natural light which is frequent 

 in zoisite. The index of refraction of zoisite seems to be higher than 

 that of a[)atite, so that its crystals stand out in relief from the slide sim- 

 ilarly to tliose of zircon, though not to the same extent. Cross-sections 

 of zoisite are also usually square, and by careful use of the micrometer 

 screw zoisite prisms niay usually be seen to be fluted or furrowed in the 

 direction of the principal axis, while a[)atite prisms display, so far as I 

 know, no such irregularity of surface. The distinction between tlie.se min- 

 erals can be drawn by one or more of these means in almost all cases, but 

 the discrimination requires watchfulness. Microlites of zoisite sometimes 

 })resent an appearance somevvhat resembling that of a rhombic pyroxene, 

 but hypersthene is as a rule strongly dichroitic, while enstatite is usually 

 fibrous and seldom if ever forms crystals. Pi'ehnite is a mineral which 

 might readily be confounded with zoisite, from which it is distinguished by 

 specific gravity and by behavior to acids. These are not very satisfactory 

 distinctions, because it is hardly practicable to test every slide with acids 

 or to obtain the specific gravity of the mineral in every specimen. A con- 

 siderable number of such tests have been made, however, and in no case 

 did either test indicate the presence of prehnite, nor has prehiiite been 

 detected macroscopically. 



Zoisite in the recrystallizing sandstones not only forms in aggregates 

 of recrystallizing minerals, but also results from the attack of quartz grains. 

 Well developed crystals of zoisite, with somewhat rounded terminal faces, 

 may often be seen growing into quartz grains from the outside almost 

 exactly as they might develop in a limpid fluid. It mast of course be suj)- 

 posed in such cases that there is a space between the ingrowing crystal and 

 the surrounding quartz which admits of the penetration of fluids, though 

 under the microscope no such opening is visible. If there is one, the para- 



MON XIII G 



