ANNIVEESAEY ADDEESS OE THE PRESIDENT. 53 



of felspar, and sometimes also with the direction in which each 

 simple or twin crystal is cut. It will thus be seen that the quartz 

 wedge furnishes us with a most valuable means for distinguishing 

 very minute crystals. It is also very useful in enabling us more 

 easily and accurately to compare the doubly refracting power of 

 different minerals, as shown by the order of the tints given by 

 contiguous sections of the same thickness, observing the average 

 result, so as not to be led into error by the varying inclination 

 of the axes to the plane, of the section. 



Mica in deposited BocJcs. 



Though the larger crystals of mica in granite or schists may be 

 somewhat altered by weathering, the microscopical and optical 

 characters are not sufficiently changed to need special descrip- 

 tion. Unless it be in special cases, when much oxide of iron is 

 present, there seems to be no other general tendency to fall to pieces 

 than by breaking up along the cleavage into thin flat plates. Except 

 when forming part of a boulder or pebble, mica could not be much 

 worn down by friction, since its shape would cause it to be so 

 readily lifted from the bottom by a current. There is, however, a 

 most important source of very fine-grained micaceous mud, which 

 I was not able to recognize until very recently for lack of adequate 

 optical apparatus. I have long known that some quartz felsites 

 might almost be called extremely fine-grained granites, and contain 

 many small flakes of mica, plainly visible with a pocket-lens ; but 

 it was not until I had found that the quartz wedge in the eyepiece 

 enables us to distinguish transverse sections of very minute flakes 

 of mica from felspar microliths that I was able to prove that, 

 though the fine-grained base of some felsites is mainly felspathic, 

 yet that of others contains a very considerable amount of mica. 

 This occurs in the form of minute flakes, varying in size down to 



about -g-gVir of an inch in diameter, and 20,000 ^^ ^^ ^^^ i^ 

 thickness. Such flakes are of course far too small to produce any 

 very marked difference in the natural appearance of the rock. The 

 decomposition of such a felsite would yield a very fine-grained 

 mud, composed not merely of kaolin, but also of mica, or at least 

 of some mineral having very similar mechanical and optical cha- 

 racters. 



Augite, Hoynhlende, and Olivine in deposited Rocks. 



Much of what I have said respecting the internal structure of fel- 

 spar will apply equally well to augite, hornblende, and olivine. Little 

 could be learned about the origin of any particular fragment unless 

 it contained well-marked glass enclosures or bubbles of gas, which 

 would point to a true volcanic origin. Not much reliance could be 

 placed on the mere absence of bubbles, yet their complete absence 

 would make it more probable that the material was derived from a 

 solid volcanic rock and was not a true ash. 



If decomposed by weathering near the surface, augite and horn- 



