224 ‘FIELD COLUMBIAN MusEUM—GEOLOGY, VoL. II. 
color of the rock, as seen megascopically, is due. That the rock is 
impregnated by hematite is further shown by the fact that when a 
fragment is heated it turns black and becomes magnetic. Slender 
veins of the same hematitic character run here and there through the 
mass. This impregnation by hematite is confined to the ground mass, 
the feldspars being largely free from inclusions. Angular outlines char- 
acterize the feldspars. Cleavage lines are not prominent, and the 
crystals do not exhibit zonal structure. They are generally fresh 
and free from inclusions. As compared with the rock previously 
described, plagioclase is more abundant. Many individuals show 
polysynthetic twinning, and there are numerous intergrowths of 
sanidin and plagioclase. The only femic mineral of importance vis- 
ible in thin section is hypersthene, which occurs in the form of coarse 
grains of a yellowish brown color. It is usually closely associated 
with the feldspars, and often is included by them. The grains lack 
crystal outline, and seem to have been absorbed to some extent. 
They have high index of refraction and little or no pleochroism. 
An analysis of the rock, by Mr. H. W. Nichols, gave the follow- 
ing results: 
Analysis of Rhyolite, Dellenose, Cerro Mercado. Mus. No. E 8182. 
ST Os. Sew ba ER eee a ee 66.32 
AlO 7s als here ihsee cee aetna es eames 15.06 
eens 6 Bog AS REAL etn sa Bs he ioe Pag 4.59 
10 Ree enon oh a ieee os ite Ed ak Oss3 
MnO. 2 pAlb RR ae sn ae imine ee es Trace 
CaO Foe Se ea oy See ee ea oe L.62 
MeO) 5 ob cui le se ite amen eere ye °o.86 
| GLO Oe Be Pee ey be chk po een 6.82 
IN 2, Ose ips Sully tee ets ete Maced sate earth re 
EO wale Jee eee arene me near aces Siter geet kas Tee 
£205: Raat au Pi oe arty oo ear aed ae CG 0.18 
Eo ah ent Pip ran Tee tat Ay Biren ie ARE Te 0.05 
DD make Tyce Oe AR Ge) Coa OR Ts a ena Orig 
PRO. Meas Sunn) ae eR ORI OF i rok thy paces 0.09 
Gia G FRAN ST elle ye eg. ee a eee 8 0.03 
ZNO eter tes TOM ene Oe Te A Be Ne Gra 
OGO Tere ee pee Ce RR ss Dia er ced a. ct eee O.15 
BE tate Mas tater ey eae, Bere ei akin Slee None 
LED 20 CU ORR etter theta St eet eae None 
2 LG Berea wren, he AOS blak: ah) eae a Gi Trace 
CTO) 5 Petes ae Sita ly REM ge wok OR, a None ~ 
