Vol. 4] Osmont. — Geological Section of Coast Ranges. 



47 



SiO, 



Al.O, 



Feo0 3 



Feb 



MgO 



CaO 



Na»0 



K.,6 



HX> + 



H,0 - 



Ti0 2 



P..O-, 



MnO 



SrO 



No. I 

 58.44 

 17.0(5 



1.36 

 5.06 

 2 . 96 

 5.82 

 3.40 

 2.84 

 2.12 

 .38 

 .15 

 1.41 

 .50 



No. [I 

 63.12 

 16.13 

 3.53 

 3.65 

 1.86 

 5.04 

 2.78 

 1.08 

 .93 

 .97 



trace 



.39 

 .38 

 .03 



Total 



101.50 



99.89 



Basic Secretions. — Macroscopically the basic secretions are fine 

 grained dark rocks, containing, apparently, a much larger pro- 

 portion of ferro-magnesian minerals, and being of greater spe- 

 cific gravity. They vary from medium to very fine grained. 



Microscopically this rock is seen to be holocrystalline and fine 

 grained, and made up of a plagioclase feldspar and green horn- 

 blende in about equal proportions. 



The feldspar, though later than the hornblende, occurs in 

 fairly well shaped lathes, almost always showing albite, and fre- 

 quently also pericline and Carlsbad twinning. Several sections 

 showing both albite and Carlsbad twinning, and cut perpendic- 

 ular to (010), were recognized, and these were utilized for M. 

 Levy's determination by concurrent angles. One section gave 

 the angles 28V2 and 14°, corresponding to a section of labra- 

 dorite (Ab 2 An,) cut at an angle of 20° with (101). Another 

 gave — 25%° and — 17%°, showing labradorite (Ah, An,), 

 making an angle of 10° with (101). 



Fouque's method for sections, cut perpendicular to a bisec- 

 trix, gave an angle of 23° 15', which corresponds to a medium 

 basic labradorite on the >i„ curve. 



Acid Dikes. — Dikes of pegmatite, aplite and greisen are 

 abundant, but no microscopical study was made of them. In the 

 field the pegmatite was seen to be a very coarse grained rock made 

 up of large crystals, up to more than an inch in diameter, of 

 orthoclase quartz and muscovite. One section of the orthoclase 

 afforded a beautiful illustration of cataclastic structure. 



