74 
The analyses (Nos. I and II) of two varieties of 
the essexite from Mount Johnson can be readily calculated 
out so as to show the quantitive mineralogical composition 
of the rocks. 
The calculation of the mode*—or relative proportion 
of the minerals actually present gives the following result: 
Essexite Olivine-essexite 
(Analysis I.) (Analysis IT.) 
Mount Johnson| Mount Johnson. 
AUD AEG ian es eee Pe eee 36°75 29-14) 
ANOTthites..< ee ae ee 20-237 66°45 13-Ilr 54°79 
Orthoclasess sc. a ee 9°47 12°54 
Nephelines 3 ree cet 3:99 E = TTR ; 
Kaolin: sree to sere ee eo -78 4°77 -78 eee 
PYTOXENE 4454.5 te Mean ee oe 6:29 12-22 
Hormblende neat noe tree: 7°05 2°30 
BIOUEC. Atos cate ners 2-04 4-08 
Olisvines: 65 fk heen tae ee none. 2°84 
WMacnetitesd.::, orice sone 568 2° 3°94 
Iimenite. cen eee eee 3°85) 9°53 4°47 8-41 
DATO ee Seth Wy ae are eine 2:68 2°59 
Water \(hyer))iJo5 =). Aan os ee -58 -85 
99°39 99-98 
The calculation further demonstrates that the plagio- 
clase in the case of No. I is a trifle more basic, and in the 
case of No. I] a little more acid, than Abz An, which, as 
has been stated, is shown by the optical character and 
by the specific gravity of the feldspar to represent its 
average composition in these rocks. 
The small amount of orthoclase recognized in thin 
sections also appears as mentioned in the description of 
the rock. The nepheline is in places somewhat altered 
to a mineral resembling kaolin. The’ small percentage 
*Quantitative Classification of Igneous Rocks (C.I.P.W.) (University of Chicago 
Press, 1903), Dp. 147. 
