May, 1904. GEOLOGY OF WESTERN MEXICO—FARRINGTON. 223 
From this the norm calculated in the three ways mentioned on 
page 205 is as follows: 
Composition 
Quantitative asaffected by kao- Composition of 
composition. linite and limonite. unaltered rock. 
OF SST eee ae eee eee ve 31.68 31.20 30.30 
ES TCLASC.: {ionic uve ile ye 31.69 37.69 31.69 
Re Oi oe aa aoe 29.34 207 34 Siaa4 
BPE UES. choc, uation hs I.39 te) 1247 
eerie tga eS a ae Lea iat oye ay att 
DSS ale ait gd oc .41 ace, tes 
Premerstocne. on las, eG Due T02 I.62 
War E GILG cto eis. ots PGR 423 oe 23 
MRP IALL CESS sh siwhs Pes, eae 1.46 Fite 1.46 
Ce a8 ee ee a ye RvtY: 
ene Tere Mee A. GOs eo Se er 1.69 I .30 I.30 
99-51 99-51 99-34 
In either case the rock belongs to Class 1, Persalane, Order 4, 
Brittanare, Rang 1, Peralkalic and Subrang 3, Sodipotassic, Liparose. 
The composition, as above expressed, agrees well with that seen 
under the microscope, except in the quantity of albite, little of which 
can be definitely distinguished. In fact, it would seem to be doubtful 
whether the method of calculation advised by the authors of the 
quantitative classification by which all soda is assigned to albite is 
likely to give a correct result, since it seems to ignore the fact that 
nearly all orthoclase contains soda, often in considerable quantity. 
The hand specimen collected to illustrate the rock nearer the 
mountain (Mus. No. E 8182), has a general chocolate-brown color 
and compact texture. Colorless transparent phenocrysts are scat- 
tered through it abundantly. These are generally lath-shaped, and 
show shining cleavage surfaces. Their average length is 2 mm. 
and their width 1 mm. Their hardness is 6, which property, 
together with their cleavage, marks them as sanidin. Occasionally 
similar individuals occur of larger size and less regular in shape, one 
of which noted has a diameter of 5 mm. The feldspars are the 
only minerals readily recognizable with the naked eye. Quartz is 
practically absent, so that the rock in this respect differs noticeably 
from that just described. A narrow vein of quartz scarcely a milli- 
meter in width, in one place slightly open and lined with drusy quartz, 
ramifies through the specimen. Such a vein is obviously of sec- 
ondary origin. Under the microscope the ground mass appears in 
general glassy in character, in places partially devitrified, so as to 
give a minutely granular appearance in polarized light. It is heavily 
impregnated with a reddish, opaque to translucent dust, the par- 
ticles of which lack definite form. This dust is probably included 
iron oxide in the form of hematite, and to it the chocolate-brown 
