' 
May, 1904. GEOLOGY OF WESTERN MEXICO—FARRINGTON. 2050; 
_ by Iddings as belonging to a collapsed pumice,* are many of them 
seen here in great perfection, and the structures correspond closely 
with those shown in the photomicrographs Fig. 3, Plate L., and Figs. 
t and 2, Plate LI. of the same work. 
An analysis of a hand specimen of the rock collected by the writer 
was made by Mr. H. W. Nichols, of the Department of Geology of 
the Museum. The results were as follows: 
Analysis of Rhyolite, Lassenose, Ciudad de Rocas, Mus. No. Li 1829. 
wep Mee ers DAR hE. oh Eady i. Fae ecient Se ae 6652.7 
UC) tae Sah Ae atin CLR ge AER Aca hl a 17.40 
HGLO)C. rect aah eae ana OE re caer 5 2.87 
Eas Ok Peale ed! Si et ee, ane eee G33 
UR oR ORES Bo Le en eet eG pees a: ee aaa ae I.05 
ONS ae et ai Nee ian a te ae T.49 
PEL NICE Oe i ie Bk os Dai gee Sen a te eae aa 5.81 
Fee Be OF ERE Ee OW fi ce A en a 25% 3 
Af hie rag eee he Renae OEE cee alae eee Sie jas es 
ar Amur w PE aU Was er uline ay) ees Eker NONE 
SEE aMBren Gir Ae mam ss vite, sue ok ary Mls beet: Trace 
LSS 51 TAN rag a, Cea ea A Hs on Cea er ES Hea None 
NES Gs Rie eae Ae Ae Mee ae) > Sean 0.12 
SARS nt GPR eed Tel Pea Ce AM bean (ck Le Orns 7 
SAU ¢ USES) ae a poet eee Se ae Trace 
oh gg Buen a Be dy 99.98 
“Calculation of the mineral composition of this rock from the 
analysis according to the methods recommended by the authors 
of the quantitative classification,} would give, if the writer has cor- 
rectly interpreted them, the composition shown in the first column 
below. But to the present writer it would seem more reasonable to 
interpret the excess of alumina as representing kaolinite resulting 
from the decomposition of the feldspars than to consider it corundum. 
Since the other components of kaolinite, viz., silica and water, occur 
in the rock in quantity sufficient to combine with the alumina, as- 
sumption of the presence of kaolinite seems justifiable. Accord- 
ingly, the second column shows the composition of the rock if the 
presence of kaolinite is assumed, the further assumption being made 
that it is formed by the alteration of the plagioclases, since, as is 
well known, these break down before the orthoclase. If kaolinite 
be present, however, it represents original feldspars, and a simple 
calculation serves to determine the original quantity of these in the 
unaltered rock. The result of this calculation is given in the third 
column below, and more nearly represents, in the view of the writer, 
the mineral composition of the rock than that of the first column. 

*Monograph U. S. Geol. Survey, Vol. XXXII., Pt. II., pp. 404-406. 
+Quantitative Classification of Igneous Rocks, Cross, Iddings, Pirsson, 
and Washington, 1903, pp. 187-209. 
