PHYS1S 



Extinction on section iNg with (ooi) = 2:>"3. 



The albile crystals appear to be tabular, parallel lo the pinacoid 

 (oío). The largesl dimensions are in the direction oí' the c axis, some- 

 times more l lian 3,0 mm. The shortest, parallel to the b axis, below 

 '.o mm. The crystals are variable in size. Their sections appear with 

 nicas from 0,2 mm 2 , to 8,0 mm 2 . 



This mineral appears somewhat decomposed in muscovite and mo- 

 reover its white colonr becomes turbid as a consequence oí" accumula- 

 tion of a kind of pigment irregularly distributed. 



Quartz. — This mineral occurs generally in aecumulations and ap- 

 pears lo be the lasl mineral in crystallizing. Its forms are always 

 very irregular and its dimensions variable. The sections attain an área 

 of A mm 5, and the smallest sections may be of an área below o,oo3 mm 2 . 

 The size» of crystals varying gradually between the extreme dimensions. 

 It always shows undulóse extinction. It is often cracked, granulated 

 and benl. Perhaps the fine grained cpiartz is at least partly the resull 

 of pressure. The quartz occurs sometimes included in feldspar but i l 

 seems thaí is only in the peripherical regions of the including mine- 

 rals. The optical constants of quartz have been ascertained. 



Muscovite. — Ng — Nm = o,oo/|5, Nm — Np = o,o347 ; Ng — Np= 

 = o,o3()2 : — 2V == 39°6. 



Thví plañe of the optic axis is normal to (010). 



There is a feeble pleochroism : Ng = Nm = very clear greenish 

 yellow, Np = white. 



The forms are generally irregular but sometimes in sections para- 

 llel to (001) elongated rhombic contours appear in the direction (010) 

 (001). Li this case the face (010) disappears. 



The rhombic section is not regular and the largest diagonal is para- 

 llel to Ng. The crystals always show undulóse extinction. 



The lamellae are generally benl. I have not observed any other 

 cleavagc but (001). 



The muscovite sometimes endoses a lamellar opaque mineral which 

 is nearly black. Perhaps it is hematite. The lamellae are intercalated 

 with the flakes of muscovite, parallel to (001). It includes moreover 

 small crystals of apatite. 



Twinning with (001) as composition face is common. 



The muscovite appears logether with quartz in aecumulations of 

 Ihis minera] and as inclusión in the feldspars in flakes situated along 

 llu' cleavage planes and in other directions as well. 



The greatesl section I observed in the slices was of about •>,,•>. mm 2 . 



