90 QUICKSILVER DEPOSITS OP THE PACIFIC SLOPE. 



tance, so that the edge of the grain seen in section is full of indentations, 

 each the bed of a microlite. This relation is more clearly seen by revolv- 

 ing the analyzer. In positions where the colors of the host are strong while 

 those of the parasites are weak, the original mineral is seen to extend out 

 among the microlites, while, when the relations of the colors are reversed, 

 the nucleus appears limited nearly to the inner ends of the microlites. As 

 the polarizer revolves, the visible limits of the parent mineral dilate and con- 

 tract in a very striking manner. 



In many cases the nuclei are quartz and the microlites are polv- 

 synthetic twins, which in favorable cases give the angles of extinction of 

 oligoclase. In other cases the nuclei are feldspar, sometimes orthoclase and 

 again plagioclase ; the parasitic microlites, however, again appear to be 

 chiefly oligoclase. That oligoclase should result under the same conditions 

 from the attack of quartz and of feldspar is in the highest degree remark- 

 able ; but the observations made on this slide and confirmed by comparison 

 with other thin sections admit of no other simple explanation. The process 

 of alteration does not go on only at the surface of the quartz grains. In 

 the granites quartz grains are frequently composite, the separate crystalline 

 individuals sometimes exhibiting a barely perceptible difference in crys- 

 tallographic orientation. The lines dividing the individuals must also be 

 lines of weakness, and when the fractures which take place in the disinte- 

 gration of the parent rock do not follow these lines they may often be seen 

 to have afforded opportunities for the attack .of the solutions and to l)e 

 marked by narrow bands of microlites nearly perpendicular to the line of 

 division. 



It is not always that the microlites resulting from the attack of quartz 

 and feldspar in this rock are lath-like. In many cases they take the form 

 of irregular grains indenting one another and assuming the granophyr-like 

 structure mentioned in the slide from Jericho Creek. These grains are not 

 polysynthetic, but evidently feldspathic, and are so associated with the 

 elongated microlites as to make it probable that they are of the same or a 

 closely allied species. 



This slide also contains small quantities of zoi.site and some undeformed 

 foils of white mica embedded in aggregates which have replaced clastic 



